What's not a sign someone's an asshole, but is often misinterpreted as one?

Honesty? Yes. Brutal honesty? Hell no.
If you don't want to be a twat, you should take at effort at considering how your words will impact a person, and try to take brutality our of it. Not lying, not hiding anything, but being considerate, that's, like, bare minimum

This is almost always just being an asshole. You can be honest and be an asshole all at the same time. There is a reason that people train in conflict de-escalation and how to talk to people in a tactful way.

People using "honesty" as a veneer to be brutal aren't the same as people that are genuinely honest.

I'm gonna be straight with you, apart from the music the Rocky films were not the peak of cinema. I'm sorry, but it had to be said.