aaand that's how we can tell you've most likely never actually played either of these two games. Slap City is a fantastic, silly, platform fighter with more polish than Brawlout could even hope to be at a 4th of the price. Don't write it off entirely because of simple aesthetic, or assume Brawlout plays any better because of a higher budget.
I've had more fun with Slap City than any of the other non-Smash platform fighters in recent history, and this is including Rivals of Aether which is also very fantastic.
I've played quite a bit of Rivals, Icons, Brawlout, and Slap City. I understand that Slap City has Wavedashing and Teching so the melee crowd loves it, but as a casual player I absolutely hate the way it feels. I do agree that it plays better than Brawlout, but not better than Rivals or Icons. (Subjectively, based on I've watched a lot of videos on it too where others are praising it, and that's cool that people like it, but I find it really weird to play. A lot of the moves just don't look right to me, and I have to go into practice mode and look at the hit boxes to see where they're hitting to fully understand them.
Slap City has the best variety of stages and modes, BY FAR. And I have to give it a lot of points for that. But if I'm gonna 1v1 someone (Not tourney style, just "No items, you vs me" style) I would much rather play Icons or Rivals. (Rivals is the best imo)
Icons' biggest issue to me is its identity. I wish it felt more unique, and had more substance to it. But when I play games online (if there's a good connection), I know what my character can do, how most of their hitboxes work, and I can deal some damage.
WIth Slap City, I find myself almost constantly confused - "What hit me? Oh that small Axe inturrupts me (Not sure if that's a thing, but it seemed like it) and a lot of the moves just feel... off to me.
I think the creators showed a LOT of creativity with Slap City, so I hope it does well. But I can totally see why other people don't think it's as great. (It also seems more geared towards people in a younger demographic than I am in)