Is it frowned upon to request solutions manuals from the author?
Your request, as a self-studier, to obtain the solutions manual seems perfectly reasonable, and I wouldn't hesitate to make it.

It's true that your request might not be successful, because, as some commenters suggested, the solutions manual may be kept away from students to prevent cheating.  It may be hard for them to verify that you are not a student trying to cheat.

However, I see no harm in asking.