That's a good question, and the answer is probably Code Review.
I've asked a similar question on Meta Programmers, and Mark's answer makes sense:
Posting a picture of a UML diagram and asking for critiques is way too specific and unlikely to help other people in the future, and I would say in general code/diagram critiques are off-topic.
Code Review.SE is currently unique on the network in its focus on helping people with very localized, non-generalizable problems like this: I'm not totally convinced it's on-topic there, but I can definitely see why people would think that.
The top voted answer on this question on Meta Code Review seems to agree, but the other two answers do not, and are also positively scored (sometimes it's extremely hard to understand Meta). And of course the Code Review FAQ says that "Higher-level architecture and design of software systems" questions are off topic, and this Meta question points to Programmers for those.
It appears we're stuck in a loop, UML diagram reviews appear to be somewhere in the middle, but don't fit definitively on either site. Personally I think Code Review is the more suitable site, and the best course of action here would be to make a case for your question on Meta Code Review. If the Code Review community decides that UML diagram reviews are off topic, ping me in chat, I'd love to help re-start the discussion on Meta Programmers.