Why? Star Allies is not a critical flop because its collor is yellow in Metacritic XD. I mean, every Kirby tends to rank between mid 70s - low 80s. Though, to be honest, I've always thought that Kirby as a series is pretty underrated.
Also, the Switch is a hot console right now, Kirby is a well-known character, so there aren't any reasons this game won't cross the 1M mark pretty easily.