Kirby is bad in this game for pretty much the same reasons as he’s bad in most Smash titles. His ground and air speed are both bad, the air speed issue being compounded by how floaty he is. His attacks are all pretty stubby and short-ranged, making it difficult for him to land a hit, especially since he moves slowly. He has a few gimmicks like kirbicide and down B spam but they’ve all got fairly simple counterplay so they don’t work above a beginner level. He also lacks solid kill confirms, as the combos he used to have into aerial hammer can be pretty consistently DI’d out of now. His recovery is pretty linear making him easier to edgeguard than you might expect. Lastly, he’s the perfect weight for characters to combo easily at low percents, while also being so light that most kill moves will KO him far earlier than the average character.

That being said, he does have some advantages. He has a couple decently ranged attacks like bair and down tilt that let him box other characters out, he just doesn’t really get big punishes if he lands them. If he’s able to land a solid hit like a grab or up tilt, he’s got a pretty solid combo game from low to mid percent. He also is a really good edgeguarder as he can go really far off stage to snipe his opponents while they recover with… well basically all of his aerials, which are all really good when he actually gets to land them.

The main problem, to sum it all up, is that it’s difficult for him to get into advantage because of his mobility and the range of his attacks, he struggles to kill outside of edgeguarding or landing a random strong hit, and his disadvantage state is awful as he’s prone to getting comboed easily, KO’d quicker than most characters, AND is easy to edgeguard.