I'd be annoyed but I'm frequently annoyed with cashiers at grocery stores.
This, plus I'm frequently annoyed with sports-fan obnoxiousness. So to me it would be a "just get me out of here" situation. I wouldn't even have engaged long enough to tell her to move to another city (which is also a little odd, honestly). I wouldn't report her to her employer though. That would be a bit much.
No, I wouldn't be annoyed. I think it's a pretty common "joke" between fans of opposite teams that they hope the other team loses. I guess it's a little odd that it's the home team, but not that weird.
I think it all depends on her tone, and the impression I'm getting from the OP is that it was rude and not friendly banter. The grocery bags I use have my college team logo on them, which is the home team, but our main rival has lots of obnoxious fans all over the state. Sometimes the cashiers and baggers will joke with me about it while they are bagging, and it's all in good fun and not a big deal.
But if their tone took it up a notch and I felt like they were really insulting me or my team, yeah I would have a problem with it. It's just unnecessary, and not the way anyone should treat a customer.
This is what I think. From the OP, it sounds like she was being serious and not friendly at all. If she were clearly joking or being playful, it would be ok. But calling a customer's clothing stupid doesn't sound friendly at all.
That's crazy! I was in a grocery store during half time and once the game came back on, it was blasted over the loudspeaker of the entire grocery store.