My favourite thing about Kirby games is that they are Kirby games.
Okay, but real talk, my favourite thing about them is probably just how goshdarn approachable they are for all people, and how surprisingly multilayered they are. They're typically very simple and fun to play through, visually and musically appealing with straightforward and intuitive basic controls, but those who seek challenge can easily get them, while not having to compromise those less skilled from enjoying the game. Not to mention just how varied everything is and how fun it is to just mess around and try new abilities.
But the other thing is just how deceptive these games can be. A friend of mine once described the entire franchise as best being represented by a Scarfy:
You encounter them as above, and can easily defeat them without ever having to see their true appearance, but once you really try to swallow them, it shows all too well.
My favourite Kirby game is Kirby's Dreamland 3 for this reason - I think it is easily the epitome of deceptively simple and cute yet surprisingly dark and complex (and not to mention cryptic as hell). The controls are simple, the graphics are just a joy, the controls are straight forward and intuitive, the music ranges from sweet and memorable to strange and atmospheric, and yet those looking for a challenge can easily get it by trying to properly clear all the levels and their respective puzzles, which are not only a challenge to solve, but even simply survive long enough to get to. And while a lot of people say that Zero's presence in the game wasn't cool for something E-rated, the game was designed so that young kids wouldn't have been able to encounter them anyhow, since again, the puzzles were so goshdarn cryptic.