Honestly, I'd say the worst pokemon is Snom. There's two reasons for this.
First of all, the design. It's absolutely hideous. It looks like it has an ass for a face, and the black dots that are supposed to be its eyes are also ugly. The shape of it is kinda ugly too, and while a shell of ice spikes is cool conceptually, with everything being ugly about snom, it ends up looking ugly on it too. Admittedly I'm not the biggest fan of bugs to begin with, but a lot of bug type pokemon actually look cool, and some of them even look cute. They managed to make a tick adorable, so they could surely have made whatever this thing is based on a cute pokemon. But not only is it ugly, but no other bug types are anywhere near as ugly as this thing (except maybe Pheromosa). It's the second ugliest pokemon behind Medicham in my opinion.
But that's not the only reason it's the worst pokemon. It's also completely terrible in battle. Of course, it's an unevolved pokemon, but even for those standards, it's terrible. While I've never actually tried to use it in battle or in gameplay ever because of the design, it's pretty clear that it'd be one of the worst pokemon in the entire series to use. It has 185 BST, the fourth lowest in the whole series behind only solo form Wishiwashi, Sunkern, and Blipbug. It's best stat is a 45 special attack, and it has 30/35/30 bulk and 20 speed, all on a pokemon with one of the worst typings in the series. It has TWO 4x weaknesses to Fire and Rock, with additional 2x weaknesses to Flying and Steel. Since it's designed to be one of the worst pokemon in the series, it has an extremely limited movepool too; outside of tera blast, it only learns two moves by level up, Powder Snow (40 BP) and Struggle Bug (50 BP). Its only other ice attack it can learn is Icy Wind (55 BP without even 100% accuracy), and it has virtually no coverage whatsoever, with its only attacks outside of its typing being Fairy Wind (40 BP), and Facade (70 BP, but goes off of its 25 Attack). Its only decent move is Bug Buzz. Considering all this, you'd think this is an early route mon with a quick evolution, but nope. In SWSH, its earliest access is in the middle of the game, and in SV, its only in the northern section of the map, where all the badges are either mid game or late game level-wise, and its caught around the 40s (and therefore far too overleveled to be used for the earlier sections of the game). By this point, literally anything else is a better option than Snom. In SV's case, you can even just catch Frosmoth itself in the same exact places. Oh, and just to make things even worse.....it's a friendship evolution. Meaning you have to actually use this thing for quite a while (and also make sure it doesn't die at all during that time) to even evolve it. And it also only evolves at night, just to make it even more complicated. All of this for a pokemon that isn't even particularly good battle wise.
It doesn't really get any worse than this. One of the worst designs in the entire series, and pretty much unusable in battle, all while still being hard to evolve.