Whoops. Meant "worst" where i said "best" in my post about jiggs vs. roy. Hence my confusion. Whoopsies.
Anyways, the marios are real tough for pika. Chaingrab hurts, and mario can fsmash or usmash (depending on where you DI) you at the end of it (like 100% if you don't make it to an edge, and you die to the smash if you don't). Doc can either dsmash a little earlier, or rack up like 20% more and dthrow->fair when he gets his next grab. And grabbing pika isn't all that hard for him. Pika has to be weary whenever he's on the ground, as jab leads to grab whether it's shielded or not. On the ground, jab and dsmash are both pretty safe options for the marios, as pika gets enough shieldstun and pushback from each that he can't punish either oos. Approaching from the air, both marios' utilt beats everything pika has if timed well. He can also dashdance->punish approaches (but almost everyone can do that, its just the marios punish with grabs/dsmashes usually and those kill pika).
Even when both chars are in the air, pika has some difficulty. Marios' uair is preeetty safe. His nair has decent priority too (and doc's hurts T-T) it's just laggy. Fair also hurts but it's pretty easy to avoid unless you're in stun from dthrow or something.
Pika does have the advantage while he's below the marios, as his uair beats everything they have. But then, you're mostly just juggling them with uair and trying to set up a spike near the edge. So 3% at a time as you try to position. And as long as he saves his jump until you can't follow up, he'll often be able to recover.
There's more minor stuff, but this is the meat of the match. At least vs. spacies pika can punish the crap out of them if he catches them. Pika's punish options vs. marios are fewer and more situational.