I can't see any problem (bar the loss of quality due to the processing and recompression ) as long as while rotating the image before upload, you also update the tilt information accordingly. Mapillary could still apply a different rotation, if necessary, to an image which has already been levelled before upload.
What I'd like to see on Mapillary site,is an option to manually adjust the angles of a choosen picture, based on a visual feedback, just like you can adjust direction and position of shooting. I shoot while walking, and probably shake the camera too much. As a result, the recorded tilt direction is often slightly inaccurate, After rotating the picture to zero the recorded tilt angles, it still looks skewed. Adjusting it would require user interaction, unless Mapillary could do some magic guess based on image content interpretation?