Some folks have really tremendous avatars... that make you laugh, go awww, say wow, or "Yup, that's right." Is there any way to offer a thumbs up or extend an acorn tip for them? If not, would it be possible to offer such?

You can always give acorns to someone for anything. Just go to their profile and click on "give acorns to...".

Yes, if you do it through their profile, it starts a t-mail thread with the receiver and you can add your appreciation note that way.

Or, you can always just send them a tree-mail to say "love your avatar!"