Will a Müllerectomy for mild ptosis also fix the uneven eyebrows caused by the ptosis?

The brow elevation is a compensatory mechanism to assist lift the droopy upper eyelid, so yes, the eyebrow should relax and come down once the eyelid ptosis is addressed. See following link and video.

when we evaluate the ptosis, we usually place one drop of phenylephrine 2.5% in the affected eye and see if the eyelid elevates to the desirable level. at the same time, we usually can see if the eyebrow drops as well.  

Pictures would be very helpful. The brow and the lid relationship is dynamic, as you have surmised, and changes in one will affect the other. There is certainly a compensatory relationship between the brow and the lid. In general, if your lid ptosis is addressed, then oftentimes the brow position often comes back to normal. I have however seen patients whose brow elevation has become a lifelong habit, and does not necessarily correct after ptosis repair. An in-person consultation would be very useful. Good luck!