I like MM and generally root for him. However, I'm withholding judgment on his GOATness until he fights TJ. 125 is still pretty new and thin, so it's difficult to gauge how much of his success is attributable to his greatness and how much is due to fighting in a weak division.
Personally, I hold TJ is very high regards. I think he's been criminally underrated for a long time. I'd say he's easily a top 5 p4p fighter. So if MM goes and besmirches TJ, his status would be solidified in my mind. To take it a step further, for me, MM is 3 fights away from being p4p GOAT. If he beat TJ, Cody (who's talked about dropping to 125), and the new and improved Cejudo, I'd be fully prepared to call him GOAT.