When I was young and just starting out, it was common advice to job seekers to research the company and find out all you could about it. There was no Internet in those days (yes, I was applying for jobs bashing rocks together and hunting sabre-tooth tigers) so I would go to the library and search through reference sources and newspapers. It was a lot of work. And I don't recall once that it ever did me any good. Pretty much every interview I've ever gone on, the first thing they do is say, "Let me tell you a little about this company", and they'd proceed to tell me in ten minutes far more than I had learned from hours of research.
I suppose if you applied for a job at some big, well-known company, and it was obvious that you had never heard of the company or didn't know what they did, that might make you look ignorant. Like if you applied for a computer job at Microsoft and it became apparent that you had never heard of their company, I'm sure they'd wonder how much experience you could have with computers. But for the typical small to mid-size company, it wouldn't be a surprise if you'd never heard of them.
That said, it certainly doesn't hurt to do some research and find out what you can. Other than wasting some time, it's hard to see how doing research could hurt you. And you might learn something that brings up questions that you want to ask during the interview. Last time I was job-hunting I invested a few hours in researching a company on the Internet before going on an interview. Not weeks, but a few hours.