Pokemon has a long history in the gaming industry, with Red and Blue releasing in Japan in 1996 and in the USA in 1998. It's come a long way, and there's little doubt that Pikachu is a household name even for non-gamers. However, such a history will mean that a few designs did not land with fans and, for the most part, may have fell to the wayside.
Some take it a bit further, being rememberable only for how ridiculous the Pokemon's design is. Clearly, some designs will have better quality than others, but for this long-running and fan-favorite franchise, these are the bottom of the barrel.
The Vanilluxe Family
Vanillite evolves into Vanillish which then evolves into Vanilluxe, an Ice-type Pokemon first introduced in Gen 5 that is based on ice cream. It goes from a small cone of ice cream to a bigger one to a double scoop, which is as basic as it is lackluster. Frozen ice cream is all this Pokemon is. Now, it would likely be a tasty Pokemon to eat, but that doesn't necessarily and probably shouldn't translate to design.
Klefki
Klefki is the only Pokemon in its family and is a steel/fairy-type based on a keychain. Granted, for a keychain Pokemon, its design isn't the worst, but the very concept of a keychain Pokemon is about as rudimentary as it gets. It would seem that having a more involved evolution could have helped this lackluster design, but alas, that's neither here nor there.
Milcery
Alcremie was recently introduced in Pokemon Sword and Shield, and all things considered, it's a unique Pokemon. It has an interesting G-max form that turns into towering cake, and its color design is based on a number of factors related to its evolution method. Yet, its pre-evolved form Milcery is everything it is not. There's nothing unique about the Pokemon itself, aside from the aforementioned evolution method, and it resembles a drop of cream or milk. If it wasn't for Alcremie's uniqueness, Milcery would be largely forgettable.
Sawk & Throh
A number of Fighting Pokemon are humanoid by nature, which has never been a big issue. Machop's line is overly muscular and has a few distinctions that make it standout, whereas others like Hitmonchan at least have a fresh take on humanoid. However, that is the not the case with Sawk & Throh, and while these are technically two different Pokemon, their design scheme is practically the same. It's impossible to mention one without the other as a result.
Both are humanoid fighters with a few distinctions: some marking on the face and lined bodies. Yet, what kills these Pokemon designs is the Karate Gi they are both wearing. Removing it in choice of something else, something that captures the whole Pokemon less and dehumanizes them further - Pokemon wearing clothes is so unusual - would make the design all the better. In the case of Machamp, its championship belt is undeniably odd, but it's a small detail that doesn't take away from the overall design, unlike Sawk and Throh's karate gis.
(Some of the) Pikaclones
Every generation introduces a slew of basic Pokemon, meaning players can usually expect to see a unique bird, a canine and sometimes even a feline Pokemon. Moreover, thanks to the popularity of Pikachu, players can also expect a new rodent "Pikaclone" most generations. Like Sawk & Throh, it's hard to not mention one Pikaclone without mentioning another, so Pikaclones deserve their own spot. Some of these were received well enough, such as Plusle, Minun and even Sword and Shield's Morpeko, but for the most part, Pokemon like Dedenne and Togedemaru just seem like ripoffs that fail to make any serious splash.
Pokemon Sword and Shield are available now, exclusively for the Nintendo Switch.