MaoMao is great because the writing is sharp as hell and the characters are fun.
The main character is incredibly dedicated to being a hero and is always serious, but that part of him is played straight and sincerely, with a lot of the humor coming from how frivolous or naive everyone around him is. At the same time, he has a ton of insecurity issues and his need for validation is both endearing and hilarious.
Adorabat and Badgerclops are both great foils to bounce off of, with the former filling in the role of hero in training like KO, but since she's not the main character, it works a lot better because she doesn't warp the rest of the cast around her like KO does. Badgerclops is a rather simple straight man, the closest thing to an audience self-insert, and his laid back slacker attitude and overly sensitive nature make for great chemistry with Mao Mao.
The giant monsters are treated as big threats and the show takes a lot of pleasure in not-so-subtly having Mao Mao straight up kill them, while the recurring villains the sky pirates are always dispatched in lethal ways only to be immediately retconned with an offscreen "We're okay...."
Most of all though, the writing and animation are sharp. It doesn't have the best animation, that award easily goes to the new TMNT, and Super Hero Girls is ahead of it too. But the animation is fluid, expressive, and fun, with characters staying on model even in the fast-paced action scenes while being expressive and lively in the other scenes in a way that isn't just aping anime aesthetics.
The jokes are good because they're not just coming in fast, they take time to set things up and pay them off. Look at Mao Mao's ukulele song or the way he fooled the fox, then freaked him out some more. Shows like OK KO can feel like filler with weak jokes to pad out the run time, but Mao Mao feels like it's been cut down to hell to cram everything into 11 minutes, and what's left is all high quality.
Mao Mao is great.