So I played a good Pikachu today. He is incredibly annoying to fight. ROB has a lot of tools to combat some of his losing matchups like Captain Falcon, Fox, and Zero Suit. He can spike relatively easy, and can at least harass offstage with gyro, nair, or fair. Gyro is decent and catches landings. He can juggle them. Against Pikachu all of ROB's perks are completely thrown out the window. I'd say it's one of his worst matchups (along with Villager and Sheik).
Pikachu offstage is just menace. Air combos are staple to this matchup, and ROB doesn't have any quick airs, besides fair, that get him out when he's in trouble. Thunder is surprisingly effective against ROB's slow up-special, seeing thay he is stopped right in his tracks. You either hit the thunder, or waste gas while the thunder dissipates. It's rather tedious. Is there any way around thunder, besides playing the waiting game, that ROB can do?
Pikachu's projectiles trade with ROB's lasers, which is nice. Floored gyro does stop jolts, but it usually not enough to stop Pikachu from advancing. Fast fall nair does work relatively well, if you space it to not hit the advancing jolts/ the jolt Pikachu is admitting. Projectile battles are pretty even, but ROB get's cornered to the ledge very easily in these projectile battles.
ROB doesn't really have anything to beat Pikachu's quick attack, though. Shield or down smash trade are his only real options, because his tilts/smash attacks are both difficult to time against quick attack and might miss completely. Dsmash's hitbox doesn't hit very high, so that misses too, albeit not quite as often. Pikachu can reset to neutral whenever he wants, harass, mess up ROB's recovery, etc with this one move.
As far as stages go, FD, SmashVille, and Town and City are bad (due to jolt/QA being much more versatile on wide open plains opposed to conjested platform stages). Battlefield, Lylat, and Dreamland also are meh, because of easy fair combos/ importance on offstage play. Dreamland's platform height/spacing might be beter than BF for ROB, though. So, yeah, starter stages are pretty great for Pikachu. Luckily a lot of the counterpick stages/ obscure stages are pretty good for ROB. Delfino, Halberd, and Castle Siege are all good ROB stages.
Pikachu should not be slept on, and I advise ROB player not underestimate him, due to his lack of kill power and light weight. If Pikachu get's the first kill, the match is probably already over.