I concur with that.
In my days as a microwave seller/repairer (not that long ago!) the better makes like Toshiba & Panasonic supplied optional extra mounting kits.
These invariably created a gap of at least an inch all around plus grilles in the front top and bottom.
I suspect that MH manufacturers try to cram too much into limited spaces which inevitably restricts the vital ventilation required for microwaves.
Easy access to the mains socket is also recommended, too often the socket is behind the oven and inaccessible in an emergency.
In one house I went to, the MW was mounted high up in a cupboard with the cable brought out through a small hole (the plug had to be taken off).
That lead down to a socket behind the upright freezer.
It had been plugged in when the freezer was empty - with a ton of frozen food inside it was totally immovable!
I had to tell the lady that she would need a carpenter to remove the cabinet and a weight lifter to empty and shift the freezer before I could mend the MW!!