The thing is even though other games in the series have scored higher, people seem to forget, that the sites that aggregate reviews just give an insight on the averaged opinion of a group of "experts on the matter" or "the audience".
And even then while the sites may assign a "consensus", the topic is at times far from being uniform in terms judging a product. For example reading a few reviews with similar scores, you have plentiful opinions about the same game, some thought it was outright bad, some that it was lackluster, some that it was fun but a bit on the short side, others that it was lacking "something", and others that the mechanics broke it.
Having played most kirby games myself except this one, i can tell you that the kirby games have mostly always been considered easy or short at some time. For example i remember this one guy in the SNES to N64 days rambling about how kirby could trivialize a lot of levels in his game by just floating from start to finish.
Or the guys a few years ago banging their heads against a wall because you can't die in Epic Yarn.