i think they are but also very few have been mapped so far. however, looks like the belde do have precise borders as long as it's know which mahalle they comprise. for example https://tr.wikipedia.org/wiki/Büyükkarıştıran#Mahalleler

OSM doesn't have most mahallesi but i think i found all of them elsewhere. now it's a matter of determining if for all belde it is know which mahalle they are made of