Not necessarily. I would say it depends. Some girls are just nice to people in general. However, if she is being extra nice to you for no apparent reason, then it is likely she is interested.

Not all girls will act the same way when they are interested in a guy. Some girls are very open and aggressive, some girls are shy and will try to not show it at all. There was this girl I met at the gym. I actually posted about her on here a while back. For a while, she would actually pass by me and not even say hi. People on here said "of course she is not interested, if she was she would not be ignoring you!" But then as I talked to her a few more times, her behavior changed completely and she started being very friendly. And then she started showing very obvious signs that she is interested. Right now I am almost 100% sure she is interested(but I realized it wouldn't work out with her so I am not pursuing anything). At first the reason she was ignoring me was probably because I was doing the same thing! I myself often feel awkward about approaching girls I like because it just doesn't feel "appropriate" at the time. Many girls shy away from guys they like for the exact same reason.

Moral of the story: you cannot really know whether or not a girl is interested based on the way she acts.