I want to participate in pixel jam, because I like pixel games, but it has a theme. In rules there is no mandatory about using theme. I don't see anything about mandatory or permission. So I am asking can I make a game not really focusing on a theme if I don't like it and just get low points on theme category?

You can create a game that doesn't follow the jam's theme, but adherence to the theme is one of the criteria considered during voting. So feel free to create a game however you want, but be aware that you probably won't get as high a score.