I’ve never seen anyone else do this either irl or in a video but I’m sure lots of people do it. When I make a grilled cheese I mayo both slices then put both slices in the pan at the same time rather than stacked in sandwich form so it takes half the time to cook and I don’t need to flip it.
I tried the mayo grilled cheese once and I agree. Not for me at all! I like to butter the outsides and that’s about it.
Nothing that was already good has ever been improved by mayonnaise. Mayo is a topping people put on mediocre and boring foods to make them edible.