Should I make fun of how my coworker dresses?
That would be mean. Do you like being mean?
It makes me smile and it brightens up my day
You've got a cruel way to earn your smiles. Don't pick on other people's personal choices.
But her fashion is dumb.
It might not be to your taste but you don't have to wear it so stay out of it. Making fun of your coworker's way of dressing or anything else about your coworker will make the workplace hostile.