Yeah, yeah. I get it. The "I'm in college, I don't have to follow any dress code, so I'll wear whatever I want!" mentality is awesome unless you ever might want to use one of your professors as a reference later on.
Many students don't treat class time like a job. Kids come in late, leave early, disrupt class by forgetting to silence their phones. Oh, and texting during class. Oh, I can't say that I'm not guilty. But in my old age - comparatively speaking- I feel as though the students around me simply don't even care.
Messy hair and a stain on your sweatpants indicate one thing to me at 2PM: you were on a late-night bender and just rolled out of bed. I can bet that's probably what your professor thinks. That very same professor may know someone that could help you get a job out of college. Too bad they think that not only are you a slob, but that you don't care. Oh, and you're never on time.
Now. Turn it around. You come to class on time and dress in appropriate clothes. How about the kind of clothes you could go visit your grandma in. They don't have to be fancy. Think about it, your prof could be a valuable resource for you. This is the way to make a good impression without even saying anything at all.