Hilarious.
Pretty for who? By whose standards? What does 'pretty' actually mean in this context?
Comfortable? I don't think that skirts so short you always have to check you're not flashing your bum, high heels, shapewear etc are comfortable in comparison to other items of clothing.
Happy? Again, happy why? In what way?
I'm a radical feminist, but I don't particularly like seeing girls (or boys) walking around in skintight leggings that go up their bum and outline their crotch or skirts that nearly flash their bum, seeing half their cleavage, or other such things that look half-dressed. One can wear a higher-necked vest top, for instance, or a skirt that hits mid-thigh instead of the top of the thigh, and it's not going to be any warmer or less comfortable.
Skimpy clothes worn outside of a beach context draw the eye - it's what they're designed to do, generally. It's why when people go out on the pull they will be more likely to wear something short/body hugging/low cut than a sack that covers them entirely.
When I was a teenager at school, my friends and I absolutely dressed skimpily at times to try to draw attention and admiration - from our peers of course, but everyone has to see it, not just our peers. And it can make many adults uncomfortable to be put in a position of seeing parts of a teenage girl that they'd rather not see/that it feels inappropriate to see.
Honestly, I think any straight man who isn't made slightly uncomfortable by a teenage girl in a pair of shorts that show the underside of her bum, and a vest top that shows a ton of cleavage, is the real worry here. Because they're enjoying looking.