Wouldn't Angle Shooter still be better than Splash Wall as a sub for Undercover? I've always seen Undercover Brella as a weapon that likes to move around so having a sub that works best when you stay in place and have enough range to poke from away doesn't sound great on it.
I do think Angle Shooter would still suck though but it's cheap, helps with movement, and gets some extra damage on opponents which Undercover desperately needs.
Also Burst/Booyah Bomb would probably be even better on Undercover than it currently is on S-Blast 91 with it's better paint and more passive playstyle. It's still the weaker main weapon but it would definitely help Undercover deal with a lot of stuff it currently can't.