To be fair, there's nothing wrong with being critical of Mormons (or any religion really). Let's not forget that it was established canon in the Mormon religion that black people were unclean and unfit to enter heaven. Not to mention that they have some pretty unhealthy views on sexuality along with a lot of other unsavory stuff.
As for "boycotting" art, it really depends. I don't think Hitler was a very good painter. What I saw of his work is very bland and lacks anything worth discussing (oddly enough I REALLY like George W. Bush's self-portraits). But if he WAS, I'd have no problem with buying his work. Mainly because his art isn't a result or motivated by antisemitism.
With something like Ender's Game, it's very clear that his personal views are integral to his art. So even if I didn't know that it was going to be garbage, I still wouldn't go to see it.
There's also other things to consider. I like H.P. Lovecraft even though he's a terrible writer, but there's quite a few stories of his that are completely racist. The focus of his work isn't really racially motivated though, so it's pretty clear that although he was kind of racist, it wasn't a driving force for him.