This is an interesting one, because Dormammu reminded me of Galactus in Rise of the Silver Surfer. Whilst everyone hated the giant cloud, the truth is, it'd be hard to get something from this character that doesn't look ridiculous. In the comics, if I recall, Dorm has a purple costume and a flaming head... in this movie he's most a giant head/face in a spiritual dark realm, with lots of stuff moving and shifting, illusions or whatever. It's hard to explain. I'd say, visually they do the character well, but he doesn't really do much though, so I could see him potentially coming back somewhere down the line. Dunno about being an MCU "big bad" level threat though, but definitely in Doctor Strange's franchise.