Personally, I don't see this as a problem, because I don't think they're just making their villains funny, they're making them people. Whiplash isn't JUST a revenge obsessed Russian, he also cares for and even loves his pet bird. Loki's a God and all, sure, but you mouth off to the Hulk without the strength to back it up, you're eating concrete, no matter who you are. Ronan's reaction to the dance off was completely normal: If you're about to destroy a world and your only opposition suddenly breaks into a song and dance routine, you'd go, "What are you doing?" too.
It adds more character to them, makes them feel more like people: They're not these calm, cool, bada** villains all day every day, they have downtime where they just indulge in their hobbies, they're not immune to being surprised or beaten down if they make a mistake they're not prepared for. That's not to say you can't have a villain with character WITHOUT having it be funny, you certainly can. Most do. But I personally like how Marvel does it, by showing they're not the always invincible (til the finale) villain. Just my opinion, of course.
Pretty much this every good villian has something that makes them goofy or normal; Hisoka can get surprised, Balalaika gets bored, the Troupe plays card games when they are bored, hell there is a trope called villians out shopping that is about this
@smart_dork_dude: @ballsmonkey: I think because previous incarnations of Casey were muscular adults they were able to be more believable. Scrawny teen version of Casey doesn't seem like the type who could fight any of the mutants and win. He should have at least a little bit of training.
Then again he does use gadgets to make up for his scrawny physique