Okay, I feel a bit more comfortable with matchups now. I've hung out online on "Elite Smash" a bit. I dunno how good they are compared to the typical community, but that's what I'm basing my opinion on.
PacMan has issues when he is out-zoned. Samus, Mewtwo, Robin, Falco seem to cause me the biggest issues. Side-B Power-pellet doesn't block Falco's lasers anymore, and its low-enough on the ground that Samus's short-hop Blaster can go over the grounded pellet. So its harder to protect yourself against a camper.
Villager matchup is absolute cancer. Its the saddest fight in the game (but not the hardest fight). The Rocket Lloyd camp game of Villager is very strong, but it relies upon Fair / Bair to keep you out, which gets stale quick and ends up chipping at you only 3 or 4% at a time. PacMan's hydrant and fruits are absorbed, but throwing many Hydrants out seems to be the best plan (although high-risk: the Hydrant becomes far more powerful after it enters Villager's pocket). The main idea is that when Villager has pocketed a hydrant, you can simply send him a 2nd or 3rd one: Villager can only pocket one thing at a time.
I'm not convinced PacMan has issues vs other strong zoners, like King K. Rool or Simon. Simon gets gimped too easily, and King K. Rool is still a heavy, so PacMan just juggles him forever. Samus, Mewtwo, Robin, and Falco have strong and disjoint aerials, and easy combos to kill you when you mess up. The same is not true of K. Rool or Simon. Snake is interesting, I don't feel disadvantaged, but I don't feel advantaged either.
The swordmen Chrom, Roy, Marth, Lucina, Ike... fall prey to PacMan's ledge game and hydrant setups. You can only drop hydrant when you're on advantage (and you ceed Advantage whenever you do so), but if you manage to FTilt them off the ledge, that's often enough to ledge-trap them with risk of death. Hydrant setups on the ledge are extremely lethal, and the swordmen don't have any recourse. Gannondorf, Capt. Falcon, Bowser, Ridley, Pikachu, Pichu fall under the same boat and are much easier to ledge-trap / ledge-guard than other characters.
Roy's speed and KO potential though is a definite issue, and may have an advantage on Pacman as a result. The other swordmen don't seem very difficult for Pac to either gimp or zone out. Little Mac similarly has high speed and KO potential, so I've been having more issues with him in Smash Ultimate. The "Trampoline for the win" doesn't seem to be as reliable in Smash U vs LittleMac.
Mii Swordfighter has a surprisingly good camp game, and forces PacMan to run in. But I haven't had as many issues with Swordfighter as Samus or Falco.
I'm still lacking knowledge on many characters: Peach, Wii Fit Trainer, etc. etc.
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Hardest Matchups: Falco / Samus
Other disadvantaged matchups: Robin, Mewtwo, Villager, Roy (??)