Luigi's Mansion is a double edged sword. If you get a lead, you can camp Wario pretty well until he destroys the house, where you gotta try to fend him off until the house comes back up. If Wario gets the lead, he can camp you regardless of whether the house is up or not (try to take it down yourself anyways).
Also, you cannot force Wario to air release unless you grab him over an edge or on a slope (Yoshi's for example). So picking Luigi's to get easier hits from grab releases is not a valid idea unless you are trying to grab him off the edge of the floors or a platform lol.
I've thought about stages a bit more, and here's my list of good, bad, and neutral for Diddy
Ban/Do not CP:
RC
Brinstar
Castle Siege
Delfino
Norfair
Lylat
YI
Pictochat
Luigi's Mansion
Port Town Aero (If that is legal in your area lol)
Jungle Japes
Mario Circuit (If that is legal in your area lol)
Pirate Ship
Frigate
Halberd
Distant Planet (If that is legal in your area lol)
Green Hill Zone (If that is legal in your area lol)
Corneria
Green Greens
Skyworld (If it's legal in your area lol)
Pipes (If it's legal in your area lol)
Counterpicks:
FD
Smashville
Neutral:
Yoshi's (It's a hit or miss)
Pictochat/Luigi's Mansion (If they aren't playing gay these stages are fine)
Corneria ^^^^^
Castle Siege^^^^
Battle Field^^^^
Pirate Ship^^^^
Delfino^^^^
Lylat^^^^
Halberd^^^^
I know this list seems... bad for Diddy lol, but it's my opinion. I've also thought about making Smashville either neutral or Ban because of that moving platform and how it helps Wario camp even better, but I've not played a bajillion times against a Diddy on that stage so for now I have it as CP. FD unfortunately for you guys will probably be banned by the Wario player, but if it isn't you can rejoice at having a comfortable stage, although Wario if he truly busts his *** can be a ***** here too. This matchup unfortunately depends on how campy the Wario wants to be. At his campiest, Diddy loses on probably every stage I can think of, even FD. The more aggressive he gets, the more Stages Diddy can get away with having it as neutral or in the Monkey's favor. I however know better and have no problem running like a marathon man even on FD lol. So just keep that in mind, if you are fighting an aggressive Wario you can wreck him on a lot of stages, but once he plays campy it's gonna be a huge spacing war.
Also, you cannot force Wario to air release unless you grab him over an edge or on a slope (Yoshi's for example). So picking Luigi's to get easier hits from grab releases is not a valid idea unless you are trying to grab him off the edge of the floors or a platform lol.
I've thought about stages a bit more, and here's my list of good, bad, and neutral for Diddy
Ban/Do not CP:
RC
Brinstar
Castle Siege
Delfino
Norfair
Lylat
YI
Pictochat
Luigi's Mansion
Port Town Aero (If that is legal in your area lol)
Jungle Japes
Mario Circuit (If that is legal in your area lol)
Pirate Ship
Frigate
Halberd
Distant Planet (If that is legal in your area lol)
Green Hill Zone (If that is legal in your area lol)
Corneria
Green Greens
Skyworld (If it's legal in your area lol)
Pipes (If it's legal in your area lol)
Counterpicks:
FD
Smashville
Neutral:
Yoshi's (It's a hit or miss)
Pictochat/Luigi's Mansion (If they aren't playing gay these stages are fine)
Corneria ^^^^^
Castle Siege^^^^
Battle Field^^^^
Pirate Ship^^^^
Delfino^^^^
Lylat^^^^
Halberd^^^^
I know this list seems... bad for Diddy lol, but it's my opinion. I've also thought about making Smashville either neutral or Ban because of that moving platform and how it helps Wario camp even better, but I've not played a bajillion times against a Diddy on that stage so for now I have it as CP. FD unfortunately for you guys will probably be banned by the Wario player, but if it isn't you can rejoice at having a comfortable stage, although Wario if he truly busts his *** can be a ***** here too. This matchup unfortunately depends on how campy the Wario wants to be. At his campiest, Diddy loses on probably every stage I can think of, even FD. The more aggressive he gets, the more Stages Diddy can get away with having it as neutral or in the Monkey's favor. I however know better and have no problem running like a marathon man even on FD lol. So just keep that in mind, if you are fighting an aggressive Wario you can wreck him on a lot of stages, but once he plays campy it's gonna be a huge spacing war.