What you see as "hot sloppy mess" is actually what's approachable. Guys tend to try to read into a girl's appearance and try to gather what they can about her before they approach. And when she puts what the guy would consider to be too much obvious effort into it, it makes her seem like more likely to be a drama queen. It probably won't make sense to you because when you think about it there isn't a whole lot of logic behind it, but to guys, there's a correlation between effort in appearance and potential for unbearable drama.
Because you are less intimidating. Perceived as easy going and not high maintenance . On a sub conscious level.. men will feel they can easily provide for you when you underdressed than when you a re over dressed. Men love to impress and provide for women. It is in their instinct. If you are wearing Armani and Gucci all the time then most likely it is hard to please you . Thus .. men will pass on .
A lot of guys find that an attractive girl is more attractive when she's a "hot mess". Like others have said, by being a hot mess you look approachable. A lot of guys don't really care about impressing everyone so to see a good looking girl who is of the same mindset gives off a sense of relaxation, thus making you easier to approach. A girl that's always dressed to impress gives off an aura of being better than everyone else. Even a completely down to earth girl may be looked at as a prude.
you may think you look a mess but you probably don't to someone other than yourself. you're always harder on yourself than other people because you focus on too many details. to them you are probably just less intimidating when you're dressed down. its been like that in the past for me too but now I don't get hit on anymore ever lol.
I would think it's because you seem more down to earth and less intimidating when you're more natural looking (no makeup, T-shirts, etc.) Like you aren't really "trying" to impress anyone. Some guys like that factor about girls, like, it's hotter when you don't put in TOO much effort. This is my guess.