Yes, a "mental challenge" just like it might be a mental challenge for someone without my skill set to do something that I excel at naturally, yet I don't call them "mentally challenged". But that's not the issue.
There are some people who, when using "mentally challenged", are relying on your implicit understanding of those connotations to communicate something else. It is by pretending those connotations aren't there that we now have stereotypes about autism, etc. It's why people tell me I don't "look like" I have autism.
So, in that sense... I don't think it's fair. But I'm an Aspie and that renders my opinion on the matter of my own labeling inconsequential ;)
Separately, though, if you ignore some of the connotations, it does seem fair to call autism a "mental challenge".