Second Danny Phanton was not really that serous at all. Less so then Kung Fu Panda seems to be. Both shows are action comedies with simplistic plots but Danny Phanton had much more gross out as I said Fairly odd Parent style humor then Kung Fu Panda has. Really even the big bad of Danny Phantom is pretty none intimidating. Vlad while a threat is not completely serous and very childish in his motivation especially in season three. The rest of the villains less so. The only two completely serous threats are one time villains. Yes Kung Fu Panda's villains are no better but that's the point they are about the same. Though threat level they are pretty dangerous.
I give you the continuity going for Danny Phantom but it often felt tacked on. One of the best and most series action cartoons ever has next to no continuty so really it's more a bonus and not a representation of being series.
Danny Phantom is far less serous then KFP for no other reason then the type of humor DP uses.
Note I liked Danny Phantom alot. Hell Danny Phantom is a better show then the Kung Fu Panda series but in terms of which show was more serous I give that to KFP. It is just as 'remotely close' to Avatar as Danny Phantom.
CG hate and no continuity are not good reasons and all you have.