I don't know if it's all. But certainly most of the 1-stars are probably just bigots. MC is gay, a small kiss, profession of love, and holding hands are shown on-screen. If you think that's going to destroy your child's or your own sexuality, then I guess it's probably just not for you. For everyone else, though, I cannot recommend this movie enough.
It's absolutely gorgeous, funny, and immediately evokes a sense of "wow, that was definitely very expensive to make." I enjoy the anachronistic elements a lot, being essentially cybermedieval if that's even a thing. The voice acting and direction is very good, top notch even.
The same can be said with the art direction. It's a healthy reprieve from the ever-present almost-30 year old Toy Story style plastic looking animation. It's distinctly much more cartoony in a very good way.
Nimona was a joy to watch. It's looking like it's going to be my 6 year old's favorite movie for a while.