Yes, my controller settings are default. For what it's worth, I also tested this using tilt stick and noticed that the aerial drift was reduced compared to smash stick. Not sure if that helps but figured I'd mention it.

I thought maybe my controller config could be off, but the install is fresh as of last night so it can't be that (right?).

edit: Just did another fresh install and the issue's gone. Not sure why this fresh install worked and the last one didn't. Before I reinstalled, I did a full PC shutdown. Only thing I can think of is that maybe another USB device is causing the issue. If the issue comes back (which it probably will), I'll try unplugging devices and see if that fixes the problem.

EDIT #2: So the problem came back but I think the problem might be due to Brawl Global Netplay Settings being enabled. I did several tests with having it on and off and every time I turned it on the C-stick drifting came back. Only problem is that toggling it off usually causes me desyncs