I look quite nice when I bother to dress decently but I have got in the habit of dressing down. I wear whatever I find comfortable and don't really care if it doesn't match or even suit me. I tell myself I can look great, I have a reasonably slim figure but I just don't do it.
Should I bother?
Does it really matter to guys what I look like with my clothes on?
It does to some extent matter what you look like with your clothes on because we're likely to suspect that how you dress is an indication of how you want us to regard you. If you dress sloppily, you're less likely to be noticed, for sure, but we may take as a sign that you don't care about, not only your appearance, but your fitness, your education, your profession, your interests, etc. Would you be just as lackadaisical about a boyfriend? You definitely don't have to dress to the nines all the time, but some indication that you're aware, collected, on top of it--that's attractive.