How do Cloud and Bayonetta counter problematic customs? (Serious question, I haven't given any thought to it until now.) And what about the other DLC characters, especially Mewtwo and Ryu?
Cloud vs. most of the campy customs (Rosalina, Villager, Sonic). Bayonetta also does decently against them. Helps that Villager's exploding balloons were nerfed iirc.
Cloud:
Villager: Free limit charge if Villager doesn't approach. Doesn't care about exploding balloons
Rosa: Can kill luma effectively enough where the customs don't matter as much (it's mostly Rosalina herself and her edgeguarding that's the problem)
Sonic: Hammer Spindash can be dealt with when you see it coming. Cloud has the disjoint to keep him off.
DK's wind tornado leaves him in a pretty poor situation against most of the DLC crew as well after you shield it.
I didn't include Mewtwo since I don't know as much about him, and how effectively he can play an anti-camp game vs Rosalina, Villager, and Sonic.
Pika's isn't bad if you avoid the jab lock setup. Pika can get heavy punished if he screws up and a good portion of the cast can do that.
Other customs iirc merely situationally improve the character and aren't seen as gimmicky. Although, I think current patch Marth probably has the makings of an extremely strong character with customs, but that's my guess.