Dropping a pass is a far cry from constantly making mental mistakes. There's also different ways to get through to different kids. Some kids just tune out the yelling, while others get scared and it puts more pressure on them and makes the problem snow ball. Other kids find it motivating and use it to correct mistakes.
In Montez' case, his arrogance so far this year has been well documented, both here and the media, as well as by his own Head Coach. If you haven't noticed, Mac doesn't stand for player arrogance, especially when he's not backing it up. Montez threw 3 INTs, slid short of a first down (which led to a blocked punt and a TD), missed open receivers and made a number of other mental mistakes. If Mac undressing him on the sideline is what's needed to humble him and get him refocused, then so be it.