A girl with a baseball cap on seems fun, sporty and down-to-earth. It conveys that she's ready for anything and not worried about getting dirty in the process. Basically, the tomboy look. Something about tomboys makes them seem exciting, like they may share some of the same interests that the guys do and play sports with them, etc., while still being a girl whom you may secretly want to kiss later on. ;) Who wouldn't like a friend that you can hang out with all day and cuddle with on the couch at night? ;)
Not to say anything against girly-girls or other girl types, however. Each is hot in their own way, in my opinion!