I would agree there's something to that in the regular season, but not necessarily the postseason.
You hammer people for 4-5 games in a row and you sort of lose that natural fear. And when you lose that fear you lose the edge that comes with it. Then you walk onto a field against a genuine team that is looking for a genuine fight and you're not necessarily as prepared as you could be to get popped in the gob a bunch of times.
In the playoffs, you're going to have that fear back because now it's an elimination game and you're ready for 60 minutes of straight-up brawling no matter who's in front of you.