I think the main reason people dislike or are simply indifferent toward U is because it released within months of NSMB2. Maybe it's a bit unoriginal at times, and the NSMB aesthetic is bland, but it's definitely not a bad game at all.
NSMB U is far from a bad game. It's just simple af.
Doesn't help that it came right after NSMB2 which was already making people grow tired of the NSMB line. Add in that it was suppose to be the first party game for the Wii U's launch and people were rightfully underwhelmed by it.