Filia: Squigly is even, painwheel is at a disadvantage (especially with assists).
Bella: Priority makes her a pain for PW, better to run Squigly.
Peacock: Depends on what the opponent is running as far as assists, specifically ones that create space. If it's vertical like BE or Updo, Squigly is preferable. Should they have a horizontal space assist, like Brass or Lock, it becomes much harder for
Squigly and easier for Painwheel since Peacock only has H george and shadow to provide air lockdown, which are telegraphed, so flight becomes that much more important.
Wheel-Squigly matchup is in PW's favor.
Fortune: If she's headless, PW can run her over, and does well with head on too. Squigly has trouble regardless.
Val: One of Squigly's worst MU's. Painwheel doesn't struggle nearly as much.
Soul: Her air priority is an issue for PW, and Squigly can charge easily, which pressures soul not to zone.
Eliza: Air priority. Difficult for both.
BB: Squigly can easily punish BB if he overextends.
Double: Difficult for squigly to deal with the catheads, PW is much better.
Wulf: No idea, but I think Squigly's superior reversal options make it easier for her than PW.
Fukua: Squigly deals well at long-range, but fares badly if she gets put into clone range. Now that M Clone got changed, both have less trouble in air. This one shouldn't be the deciding factor.