Yes

If it’s something actually bad about someone that they can change pretty easily? then it’s not mean to tell them that’s the reason.

If he spent every night drinking alcohol or smoking weed and that was a problem for you? you’d tell him that was the reason.

If he was yelling at you or hit you, you’d most likely tell him that was the reason (obvious fear of him getting angry, maybe not)

Telling him he’s a selfish lover and that he’s only having sex for himself is fine. It’s something he can easily work on if he wanted to. So yea I would absolutely tell a partner I was leaving them due to them being bad in bed. (Again, don’t if you think he could get violent over it)