Bayern-fan here with some thoughts:
In general, the opinion on Mazraoui is mixed among Bayern-fans, but i think its safe to say, the majority wont be too sad that he is leaving (also because we could use the money, frankly).
When thinking about him, i also have mixed toughts. On the one hand he has some nice strengths: technical gifted for a fullback, his build-up/passing is good, he can be solid defensively. This showed when Maz had some good games against strong opponents in the CL.
But on the other hand, he had some very poor games, when even against lesser sides his flank was completly open (I remember Tuchel going crazy). Beside inconsistency his most glaring weakness is physicality imo. I see his injury proness often metioned: It's not only that, but he takes long to come back fully, and even when he is suppossed to be fit, he is one of the first players to be gassed.
Mazraoui is a mixed bag, let's see which one you will get; having Ten Hag as a coach will certainly benefit him.