It's not about the traits themselves, but how the character's world reacts to them. A stone-faced assassin who meticulously collects novelty mugs from his targets' kitchens is hilarious because he treats it with the same deadly seriousness as his contracts. The immersion holds because for him, it's a legitimate, ritualistic part of the job. The humor comes from the disconnect between the mundane obsession and his lethal profession, not from him becoming a clown.
You see this in 'The Case Files of Lord El-Melloi II'—Waver Velvet's grumpy, sleep-deprived scholar act never undermines the complex magecraft; it grounds it. His trait isn't 'being funny,' it's being a genius forced to deal with administrative hell and entitled students, which any overworked person can recognize. The trait feels real because it stems from his circumstances, not just slapped on for gags.
Honestly, the best comedic traits are extensions of a character's core flaw or coping mechanism. A regressor who is impossibly competent but has a pathological, sweaty-palmed fear of butterflies because in a past loop one was a demon lord in disguise? That's gold. It doesn't break the tense atmosphere; it heightens it by giving this powerhouse a specific, irrational vulnerability that can be triggered in the most innocuous settings.