That can be a difficult area to spot trouble, so it's great that you noticed it and are searching for what next steps would be for something abnormal.
My initial thought was also drool. Another is if her chin typically stays wet because maybe she isn't the most coordinated with using a water bowl? I know some rabbits have more around there than others, so sometimes it can stay wet and that can be a site for fungal infections (like ringworm) to fester up. Ringworm infections start with fur loss and will grow in size if not treated. It's called ringworm because it forms a ring as it grows, since the infection is moving, so fur may grow back in areas it moved from, creating a ring at times. No actual worms involved!
A last thought, which is uncommon but worth mentioning because from personal experience with my rabbit it's happened, is syphilis. While syphilis typically shows up as blisters on the private parts, there are atypical cases where there are only signs (fur loss/blisters) around the mouth and/or eyes. My rabbit had fur loss on his cheeks that was originally ringworm, and then that seemed to trigger syphilis to become symptomatic and he began getting fur loss regularly around his mouth. So just a thought, but unlikely. I think the moisture bit is much more plausible.
