Modern Western society likes having things in neat little packages. Part of that is that some things are considered "children's activities" while others are "adult activities." And, to go with that, the idea that children should be children and adults should be adults.

Sports are considered a thing for (primarily male) adults. "Dress-up" is considered a children's activity.

So, some guy wearing nothing but a pair of shorts, body paint and a clown wig outdoors in freezing temperatures is "normal adult activity" because it's for a sporting event. While dressing up as a character for a TV show is considered childish and thus reviled by most "normal" adults.

It's pretty rediculous, but it's difficult to change society...