Harley definitely loses to supes. I play both characters, and she really does suffer in the matchup. Only time it maybe 5-5 is if NRS ever bother to fix the bug that stops harley punishing supes with b22. B22 whiffs on all floaty characters and it's dumb af, her main punish sting is useless in this matchup.
Also, supes definitely isn't "mathematically" outzoned by her. It's up to harley to get in, yes absolutely you can punish supes when he does non-MB air laser, but harley is only gonna get a 44 dmg punish (96 with meter) compared with superman's 173 or 216 (with trait), and tbh only a dumb supes will use non mb air laser vs a good harley. Basically it means that when supes is in the air, he's free to either mb laser or air dash (for fear of potential laser). It's not so hard for supes to get in, but for harley it's much more difficult due to threat of MB laser forcing her to block backwards.
Been playing harley at high level since inj1, I picked up supes only recently (2 months ago). I love both characters, but there's no denying superman has a way easier time than harley does. I lose to superman as harley on the regular (as do many tourney players I know), on the other hand I rarely ever lose to harley as supes.