Japes actually isn't so bad vs. Pikachu.... it's not necessarily a horrible stage for him, but if you camp one of the smaller platforms, Pika has almost zero approach options, and Zelda can outcamp him due to the nature of her projectile vs. his.
Once he gets on you, though, you'll have a hard time getting away again, and the water screws Zelda over much, much more than it screws over Pika. Outright killing will be a problem for both characters.... thunder doesn't even reach to the top of the screen here, so KOs from it are rare on Japes.
In general, Pikachu's best stage is Final Destination. He also does well on Halberd, Delfino, PS1 & 2, Smashville, and sometimes Norfair and RC (both very character-dependent).
His worst stage is Brinstar. Luigi's Mansion used to be considered awful as well, but since the discovery of jab locking, which is pretty easy to set up on this stage, it's not nearly as bad. Still ban-worthy from his POV, though, if it's not already tourney-banned.
Tbh, Pikachu is the king of neutrality... he doesn't excel beyond belief on any stage (except maybe FD), but he doesn't do terribad on any stage either except for Brinstar. Pika mains also tend to be really diverse in their stage picks since Pikachu is pretty versatile.... some love platforms, others hate them... some love janky stages for pulling off QAC tricks & other shenanigans, etc. etc.
My suggestions would be:
Good stages (consider CPing): Brinstar*, Battlefield, Castle Siege, Luigi's Mansion
Bad stages (consider banning): Final Destination, Pokemon Stadium 1, Norfair, Rainbow Cruise
*I'm not really sure about Brinstar since I've played there maybe a handful of times with Zelda.... I dunno how well she's supposed to do there. If you like that stage, take him there as your first choice.
Like I said, though, Pikas are weird. Some of us hate certain stages that others love, so it really depends on your opponent.... especially on how campy/QAC-happy they are.
Campachus will prefer flatter/tjolt-friendlier stages like FD, Smashville, and Pokemon Stadium 1. (The sub-breed Ledgecampachu will be partial to Norfair.)
Platform Pikas (sort of a sub-breed of QAC Pikas since they can QA around on the various platforms) tend to like Smashville, Battlefield, and Lylat. Halberd and Delfino are also good for QAC tricks since Pika can QA up through the stage.
Pikas who like to pull fancy shenanigans will probably like stages such as Delfino, Norfair, Luigi's Mansion (I told you we're weird), RC, Frigate, and PS2.
Some Pikas love walk-off BS even though we can't CG Zelda. Those 'chus might take you to Delfino, Castle Siege, or RC.
Hope that helps. <3