Completely depends on the girl and the situation.
For example, I tend to look at girls who wear heels to class as having self-esteem issues. Flats can be hot, too, especially in everyday situations. Heels on a special occasion, though, are all right.
Now, the fact that heels are outrageously uncomfortable means that a girl wearing heels might not be all that approachable, but that's a different story.