I don't feel here nor there about thank-you posts. The majority of users are on the app, which means they neither see the thank-you, not do other app only users. That said, I think it's both a nice way of showing appreciation and a useful way for people to glimpse your art. I've looked at comics because I liked the thank-you picture on someone else's profile. But I'm nosy like that?
I think the only thing that bugs me about it is that app-users have no idea either way about it, which means they're not in a position to delete the thank-you's if that so desire. Most of them probably don't know they have a profile page, so anything could be posted to it without their knowledge.
However thank-you messages are pretty harmless, and I assume they do it because they see other people doing it, and it seems like "a thing" and if it happens to also have an advertising bonus, why not? Again, harmless. ((but everyone has the right to delete whatever off their pages))