I'm a Mariners fan who lived in Chicago for a few years, and went to see the Mariners at US Cellular a few times, the natives didn't take much notice of us. I wore Mariners attire, but I wasn't overly vocal (I'm generally not anyways).
Now, if you go in full Brewers regalia and make an ass of yourself, and get in their faces, all bets are off.
Also, the area around the park is kind of sketchy, and I've heard it suggested that you not hang out there after the game. We would generally leave the park pretty much right after the game, walking directly to the 35th/Sox Red Line El station. With the crowds still around, the neighborhood didn't seem a problem.