If it's actual defacing property and causing harm to property, then I agree, it's wrong. But I'd take much less offense
to a Utah flag being planted on the Y at LES than I would from BYU actually losing the game.
But to make it more tasteful, what if the planted flag also included an attached check made out to a scholarship fund or some other charity program affiliated with the rival (ex: United Way/BYU) that was funded with NIL money pooled together by the players on the team as an expression of overall goodwill towards the rival institution?
I actually think that could be a fun tradition.