When I get ready for class in the morning, I'm usually thinking about what notebooks I need to bring with, where I left my laptop charger, and if my homework is stapled and tucked away. For me, picking out what to wear is something that isn't super important. I usually put on a t-shirt and a pair of jeans, and I'm ready to go.
Recently, I've seen a lot of articles such as this one, telling people how they should dress for class. While I think that it is important not to be a distraction in class, I think that people should wear what they want. You're in college to learn, so it doesn't matter if you have on a cute dress or a t-shirt and leggings. I would much rather feel comfortable in my hoodie while attending lecture than being uncomfortable trying to dress up, and not being able to focus as a result.
As for your professors, they probably can't even remember what you wear to class, let alone have time to judge you for it. They have busy lives, and your work ethic and personality will have more of an impact on what their opinion is of you than whether you wore a suit or a t-shirt to an ordinary lecture. At my college, some of the lectures can have a hundred students, so in reality, the professor won't even know who you are unless you go to office hours.
Where I live, the winters are brutal, and for most of the winter, I like to wear sweatshirts. They're warmer than most sweaters, and I'd much rather be warm on my fifteen-minute walk to class in 10-degree weather than try to dress nicely and freeze.
Just last week when I had a final exam, I put on my favorite sweatshirt and some leggings. Did this make me any less successful than anyone else in my class? No, of course not. I was able to concentrate on my exam better and was able to get more sleep beforehand because I didn't spend much time getting ready in the morning.
While you should be able to wear a t-shirt and jeans most of the time, there are specific instances where dressing up is essential. For individual projects or majors, it might be necessary to wear a suit. However, that doesn't mean you have to dress like this every day. College is stressful enough as it is. Why make it harder than it already is by dressing up more than you want to?