My rabbit has seemed to have a slightly messy bottom too. It gets … not matted, but seems to dry wet and then be impossible to groom and fix. I feel like the only solution would be to give bunny a bath, but sadly bunnies can’t get baths as I’m sure you know. My rabbit must be part Dutch if you look at my avatar so he doesn’t have that much hair. And I can’t fix it because when I try to put his brush there, just the top little bit of the dried wet, matted fur he runs away. Also he hates being picked up, doesn’t tolerate it one bit so I am mostly positive that turning him over for me wouldn’t work either.
I would think that a regular trim actually might work, depending on how much you decide to clip.
To do it without hurting her, I would probably get somebody to help me if I need it, which would most likely be a friend if they come over or either of my parents. Then I would try to burrito wrap my rabbit and have whoever is with me pull the blanket or towel up a little bit to his bum and hold him slightly so he couldn’t jump off, then try to clip him.
Sorry I don’t have much advice, I only have this problem too and wanted to share. I’m probably gonna watch this topic so maybe I can learn from it too.
Good luck!