I'm not tall enough to reach the shelf my roommate put the mugs on
All you have to do is to ask your roommate to put the mugs on a lower shelf.
Well, I'm planning to just move out even if he does change the shelf. I'm on my own now
You should have just asked him instead of complaining to me, it should not be a scary situation. You need to show more courage because it's just a simple request, he will comply. If you don't learn to speak up you will waste a lot of time thinking about what to do instead.
I just don't know if the shelf is adjustable or not, maybe I will just ask.
Being a roommate takes compromise from both.   If a simple dispute like this is not resolvable, then you should find a new roommate situation.