Acadia was established with all sorts of weird conditions, which is why you have the free and too-popular section of the park and then the for-fee loop road. Cadillac Mountain in particular is an absolute mess and overrun with crowds (signs say "Leave No Trace" -- Seeing the unmanageable crowd on the summit, my father quipped, "Yep, leave no trace of wildlife whatsoever."). I am sure they may establish reservations for the Loop Road, but they'll probably keep doing what they've done for Jordan Pond/Cadillac Mountain -- when the lots fill up and the waiting line is jamming the driveways up, sorry, just drive around until something opens up.
I guess I’m a few days late for the OP, but having been to Acadia in July, I might as well share my experience: Cadillac Mountain was busy, but far from overcrowded, with lots of room to spread out. This is at least partly because reservations are now required, for a small fee, to limit crowds during Covid. There’s only a certain number of vehicles/passengers allowed up during each half-hour window, and you have to arrive during your half-hour to be allowed up.
Sand Beach and Thunder Hole were busy, with tons of cars parked right on the loop road, but I assume that’s probably normal. Parking was hard to find in Bar Harbor, but it wasn’t too bad in terms of crowds. I heard that it’s been considerably less busy than usual (pre-Covid) in town with no cruises stopping there.