Any villain who calls themself evil seems kind of childish or they're trying to make a point.
With the Brotherhood of Evil, they're trying to make a philosophical point. The Brotherhood of Evil Mutants kind of ended up with the name due to Marvel adapting elements of the Doom Patrol series into their universe, resulting in people who saw themselves as good but called themselves evil so children would know who too book when they showed up.
Doom isn't really the kind of character to go "Look how evil I am! Mwhahaha!" in order to make a point, thus him identifying himself as a villain seems kind of childish as he seems to be the sort of guy who makes out people like Reed Richards to be the actual supervillains.
Someone who revelled in their evilness wouldn't necessarily be someone to create good diplomatic relations, even friendships, with superheroes like Black Panther and Namor... *remembers recent characterisation* Shoot.