Some of it is subjective, no question. I don't know enough about (or, truthfully, care enough to know about) exactly what the judges are looking for in the scoring. So, for the average person, I can only take the word of the supposed experts. They all unanimously agreed Mayweather controlled the fight following round three, including the judge who was giving McGregor the most love early. That also seems to be the narrative I've picked up from several outlets this morning.
As for McGregor belonging, are people really genuinely surprised McGregor didn't get completely blown out of the water? I know MMA and boxing have a lot of differences, but they also have several similarities, and Mayweather's fighting style didn't seem designed to destroy McGregor from the outset. McGregor is a talented athlete in his own right.
I don't know. It just seems from what little I know about both guys, MMA and boxing, it always felt like McGregor would at least hang in there.