The level design for Star Allies' main adventure is pretty toothless even relative to other Kirbies. But the world maps are cool, because they change "scale" as you advance (So World 1 is just going through one region of Pop Star, World 2 is hopping all over different locations on Pop Star, then the final world is a full-on Milky Way Wishes homage, where the map consists of flying across different planets that you can take on in the order you choose).
The game got a ton of free DLC updates, including a playable character (or team of characters) representing each of the main series games (including dudes like Gooey, Adeline, and the Squeak Squad). They are each really well fleshed out, gameplay-wise, even moreso than the typical abilities (which, if you've played Robobot, you know have quite well fleshed-out movesets themselves). There's a mode that's basically a streamlined version of the main campaign where you can play as any helper character, and some of them get unique areas based on their particular abilities.
Then there's a sort of sequel campaign that's much more tailored to flexing the muscles of certain abilities/characters (each guest character, plus about ten different abilities each gets a tailored segment in this mode), and it reveals more of that Deep Kirby Lore that the series has been embracing more and more in modern entries. And it has the hard mode forms of the bosses. I've always advocated for Kirby's style of hard mode, where bosses completely change patterns and have new attacks and forms, rather than simply tweaking HP/damage values. Fittingly, there's a boss rush with a whole ton of different difficulty settings and you can tackle it with all characters/abilities.
So basically, out of the box it was maybe dragged down by its not-so-great level design, but the abilities and playable characters give you more options than any previous Kirby game, and with the free additions, there's plenty of fulfilling content to use them on (heck, there's a classic boss that is only fought during the credits sequence of a particular mode!). Very fanservicey, huge celebration of all things Kirby (the main series, anyway).