Mortal Kombat is a great game to play with friends casually because anyone can learn to press buttons in a string and block simple mix-ups with a block button. The series has evolved immensely since its original release and it shows with the extensive character roster, long list of special moves, and even unique variations for each character.
The first step to improving defense in Mortal Kombat is learning why certain moves keep landing, and what you can do about it. Mortal Kombat is like any other fighting game in this regard and sometimes it takes time to figure out the answers to these layered puzzles, but it's well worth it in the long run.

Kitana - Edenian Razors

Kitana doing Edenian Razors
In her second variation, Highborn, Kitana gains access to Edenian Dance which works as an anti-air, combo extender, and unbreakable combo route off of grounded hits. The real reason the move seems to hit more often than others is that Kitana can change her advantage on block and hit if the player times the button release right.
If a Kitana player times it perfectly, they can do an attack after the move is blocked, which makes blocking against this move scary. The best bet to beat it is to let the opponent feel secure in using it until it's the best time to strike. Timing the release perfectly is difficult and it's important to challenge them and do big damage when they mess up.

Scorpion - Hell Port

Scorpion Doing Hell Port
Scorpion's teleport is a classic move of his from the early days of Mortal Kombat. It's always been pretty cheap because of its quick start-up and the fact that it has to be blocked standing up to avoid it. In Mortal Kombat 11, Scorpion players can now cancel the teleport to appear right in front of their opponents and scare them with the potential high attack.
This move only works on players who are aware of the original Hell Port mix up because if they don't they'll just punch the Scorpion in the face for popping up so close. To beat the canceled teleport, it's best to look out for the moment Scorpion flashes white. That flash indicates the player used meter on the teleport and will most likely cancel it to mix up the defense further. This technique isn't always correct so it's best to preemptively react to the teleport with a quick launcher that can start a combo.

Sub-Zero - Slide

Sub-Zero Slide
The biggest reason the slide is so strong is that Sub-Zero can combo into it for big damage and hit people from seemingly out of nowhere with his high-low mix-ups. The best way to avoid Sub-Zero's mix is to remember that low attacks are generally faster than overhead attacks. Practicing blocking against Sub-Zero is a great place to improve defensive reactions because of his reliance on these simple mix-ups.
Once players can block low effectively against his slide, they can deal huge damage to him since it's extremely unsafe. Most Sub-Zero players will try the slide from a good distance away since it's a great way to mount offense so try to bait it out from a distance with a jump or block and punish after it misses.

Erron Black - Boot Drop

Erron Black Doing Boot Drop
Boot Drop is one of Black's cheapest moves because of how quickly the move starts up. He lunges forward and performs a dropkick overhead that can trigger a Krushing blow if it lands twice in a row. Boot Drop coupled with his Rattle Snake Slide can leave opponents guessing when and how he's going to approach. Both moves start up similarly and one has to be blocked standing, while the other has to be blocked crouching.
The best way to beat this strategy if reacting is off the table is to block high. The Rattle Snake Slide does less damage and is mostly used as a combo ender so it's the lesser of two evils in this mix-up. The Boot Drop can lead to Krushing blows if players aren't careful so Erron Black players will try to use that over and over. Blocking high leaves Erron Black vulnerable so be sure to find the appropriate punish to make opponents scared to use that move.

Sheeva - Stomp

Sheeva Stomping On D'vorah
The ultimate scrub killer, Sheeva's whole move set is filled with annoying moves that will kill opponents quickly if they don't know what's coming. Her damage is unusually high and her mix-ups are very potent up close forcing players to make decisions while on their back foot if she gets started.
Stomp is the ultimate example of this. Sheeva leaps into the air, off-screen only to slam down with an unblockable stomp attack. At first, it isn't obvious that the move is unblockable, because it seems more like it would be an overhead. This move doesn't have a straightforward way to beat it for every character.
Players have to go into the training room and learn their specific punishes since it's such a unique move. The general rule is that getting out of the way is best, even if that means no retaliation because the move is devastating if it lands.

Liu Kang and Sindel - Throw Krushing Blows

Activating Liu Kang's Krushing blow throw
In Mortal Kombat 11, there're Krushing blows that activate if the opponent techs the wrong type of throw, that being forward or backwards. Most of these activate because the character has a Krushing blow that is triggered by a specific direction of throw so they're generally easy to avoid if players tech the direction of their Krushing blow throw.
Liu Kang and Sindel are different because their Krushing blows activate in both directions which makes them exceptionally cheap. The best way to counter these is to crouch to avoid the throw, then strike with a launcher to get big damage. This can be risky because both characters have means of punishing crouches so it's best to save that tactic for when the opponent is fishing for a grab.

Sonya Blade - Leg Breaker

Sonya doing leg breaker on D'Vorah
Sonya is known for her tricky strings and specials that lead to a lot of high-low mixes and Leg Breaker is no exception. Leg Breaker triggers a Krushing blow if the opponent stand blocks it, leading to a whopping 30% damage.
There's no real trick to blocking this move properly but if it's blocked successfully, Sonya is at a huge disadvantage and should be punished accordingly. Most players will try to sneak Leg Breaker in at the end of a string that forces their opponents to block high, so to play it safe, block high and right when the string is over, crouch block and try to react to any overhead that might be coming.

Jacqui Briggs- Hyper Knee

Jacqui Briggs Squaring Off
Generally, this move isn't allowed in competitive play but is fun for custom variations. Jacqui lunges forward and does a mid-knee strike. The attack itself does a lot of damage and can be done in combos for Krushing blows and extra damage.
The move can be baited out from certain distances and is very punishable if blocked. If the Jacqui player sees the move is blocked, she can cancel into a second hit that makes it safe. To counter the enhancement, players have to duck and then they can avoid the strike and punish with a launcher.

The Terminator - Terminate

Terminator DLC doing Terminate in MK11
Possibly the cheapest move in the game, Terminator spends a bar of offensive and defensive meter to gain infinite super armor for a period of time. The time frame feels a lot longer than it should be and Terminator has access to so many mix-ups in his high-low game and command grabs that deal crippling damage.
Terminator's movement is restricted in this mode so he can't dash, jump, or block. The best bet is to take the lesser evil of his mix-ups depending on his variations and run away until the timer runs out on his armor since he's restricted to slow movements.

Shang Tsung - Shake

Shang Tsung performing his shake parry as smoke
Shake is a move with great versatility that's also easy to do a lot, and get rewarded for it. Shang Tsung turns into Smoke and vibrates, enabling him to parry any projectile. If the move is enhanced, it also parries basic attacks. On top of all this, if moves are parried twice in a row, he gets a Krushing blow.
The good news about this move is that it loses to sweep and low attacks, as well as attacks from the air. To beat it, stay patient and watch when the Shang Tsung player likes to lead into an opening for a parry. Showing the knowledge that it loses to low attacks often deters them from spamming it.