Yeah, Mansion would suck. I don't know any really good Diddy Kongs, so I tried looking on the Diddy Boards for clues on what stages piss them off (kinda how everybody knows we hate Frigate). The only thing I could get was that Diddy HAS to have momentum in order to do well, which we all already know. But, basing it off of that, here are my picks for stages that would probably be bad for Diddy (some regardless of how awful we would suck there and this is just me brainstorming):
NEUTRAL:
Battlefield
NON-NEUTRAL:
Delfino Plaza
Rumble Falls
Pirate Ship
Norfair
Lylat Cruise
Castle Siege
Distant Planet (which I'm pretty sure is usually banned)
New Pork City (which should be banned because of size)
Summit
75 m
Hanenbow (?? It'd be really hard to pull off a Banana Lock ??)
Green Hill Zone
Corneria
Rainbow Cruise
Brinstar
From those, I think our best stages against Diddy would be:
Battlefield
Delfino Plaza
Norfair
Lylat Cruise
Castle Siege
Summit
Green Hill Zone
Corneria
This is just my brainstormed ideas, so I know they're not best. What stages are usually banned, both at tournies and by Diddy mains? That list could help me personally a lot.
Edit: I just tested out Hanenbow against a level 9 Diddy (not a good method ata all, I know, but a level 9 Diddy is one of the better cpu's) and I think Hanenbow would be a great Counter Pick. Nearly impossible for Diddy to gain momentum, Squirtle is sooooo much fun to use here, and as long as you don't try to recover near the middle of the stage in the tree part, especially with Ivy, you should do great, I think.