The reason Kill Uncle took so much stick at the time is that it was the first album after the near-perfect output of the Johnny Marr and Stephen Street era (I and quite a few others consider Street's stuff to be almost the equal of Marr's).
Kill Uncle was clearly not in the same class.
But viewed all these years on, and compared with the constant patchiness (and one might argue mediocrity) of the post Vauxhall albums it really isn't that bad at all. It's got 3 or 4 great songs on it which you can't really say about many of the post-Vauxhall albums. Also, as others have noted, the arrangements let some of the songs down.
It got a critical mauling for the same reason as Ouija Board.
Ouija Board is a pretty good Morrissey single viewed from 2016. But when it came out in 1989, it was arguably the first Morrissey or Smiths song that wasn't dazzling (or at least very good) so it got savaged.
But it's clearly as good, if not better, than many of the post-Vauxhall singles.