As palmerfan said, Pika can do ok vs Marth (depending on how comfortable you are with pika), and he does pretty well against falcon. I don't know Ganon vs Marth, but I think pika does pretty well against Ganon too. He probably also does comparably well against spacies as marth does. Other than odd low-tier matchups though, that's probably it.
I don't know a whole lot about pika vs yoshi (my yoshi experience is very outdated), but you have to respect his super armor. You will get wrecked if you don't. Pika dair is decent in this matchup because it will break his superarmor surprisingly early. I suspect dsmash (especially while behind him) should be decent too, because he can't reasonably parry it and can only get grab out of it if you're in front of him, and even then getting grabbed by yoshi isn't tooooo bad (though you should avoid it obviously). Just respect Yoshi's ability to take a hit/parry and punish you off of it. Go for more grabs than usual, hit him with uairs from far enough away that dj nair won't hit you, etc. And recognize that he can combo you really well, figure out how to DI his hits to give him the least followup options.
Against Samus, as dkuo and others mentioned you need to really be weary of her cc ability. You can't just charge in with nair like usual. Uair is decent because it breaks cc pretty early, and dair is actually ok for similar reasons. You also can't just hang back, though, as missles are tough to handle. Pick your moment, get in with an empty hop->grab or shffl dair, and get samus into the air. You can combo her a little due to her weight, but she is fairly floaty and nair will break her out of the combo if she's given any time out of hitstun. Just try to end your combo by knocking her off the stage or by staying below her and close by. Her aerials have decent hitboxes, but none of them are particularly safe. If she throws out a bomb/aerial to try to catch you, dodge and counter its lag with a usmash or aerial (or whatever option works for that situation). If she doesn't, either catch her with a uair anyways or put pressure on her when she lands, just remember her cc.
As for edgeguarding samus, jolts are pretty good if she's bomb jumping. If you hit her, great. If you hit one of her bombs, also great. Jolt can hug stages as well to potentially catch her hanging from grapple (though in most cases samus will opt to immediately retract to ledge). Ledgehogging works ok against samus with proper invincibility refreshing because you can throw a jolt or jump out after her and get her to use her second jump, limiting her options, and then ledgehog->nair/etc.
Ganon is ez. Sorta. Don't get hit. And, like you said, get under him. If Ganon is using an aerial, respect it. In fact, if Ganon is doing a move, respect it. You're not going to beat his anything, and even trading is a horrible thing the majority of the time. Fake approaches and throw jolts to make him have to react, and then get in afterwords. As soon as he lands is a pretty good time to get in, because he doesn't have a particularly good defensive option immediately upon landing. Just keep in mind that jump aerial -> double jump aerial right before he lands is a thing Ganon can do, try not to fall for it. Once you're in on Ganon, kill him. It's kinda similar to Falco in that regard I guess? It can be tough to get in on Ganon, but once you're in with Pika, like, he has soooo few decent ways of getting out. As long as you watch out for his bair and uair, recognizing how long he'll be in stun etc., he is combo food. And he shouldn't be able to recover against you. So be patient, try to find or create the right moment to get in where he doesn't have good defensive options, then stay in if you get a hit and get the hell out if you don't.
Oaty
i think most pikas agree its at least slight samus
