There is a question as to how effective self-promotion threads are. It can be difficult to get clicks, but I can say I discovered some hidden gems I'm not sure I would've found otherwise.
As to whether or not it's "polite". Well. I have seen people silently link their comics seemingly without even reading the thread (i.e. "link your own comic and rec another" and they only plug their own).
Other than that, I'd be a hypocrite for saying self-promoting bugs me, hah. People are just excited to share their work.