Childrens neurological systems require different forms of input to regulate themselves. Some kids require more movement activities (running, jumping, hopping, swinging) and others require more fidget type activities such as your daughters. As adults we "usually" get these under control, but when stressed or sometimes bored we engage in these behaviors - as adults it may be tapping a pencil, shaking a leg, rocking slowly back and forth, biting our nails, etc - things that are generally "acceptable". A good book, though I am not sure that your daughter fits any of the categories in it is called The Out of Sync Child by Carol Kranowitz - there is also another called Quirky Kids but I don't know the author about kids who are a little unusual.
If your daughter wanted to substitute the sign language with a different type fidget behavior, a water bottle with the lift top lid that you have to suck out of, is often a good substitute because it is age appropriate and the suck/swallow used can be relaxing to the nervous system.
Other kids are not very nice sometimes. She is at the middle school age when social stresses become heightened. The sign language could be a way for her to release some of the stress. It is so hard to encourage our kids to not worry what others things when the world dictates how we should look, act and feel - and often the worldly ways are way off!
Aside from being made fun of (which would break my heart) I would probably let it go and figure that it will resolve itself when it is no longer needed.
There is a lady at our church who is a sign language interpreter and I notice her "interpreting" the songs or the message when she has no one by her who needs sign language to understand?! I too, thought it odd, and then decided that was her way to participate in worship.
Good luck to you.