I wouldn't assume that every girl is like this, but I can tell you that there are a lot of girls who would laugh at all your jokes, even the stupid ones, if they like you. It's not a guarantee that she likes you, but this girl at least has an interest in you. People, not just girls, usually laugh at other's stupid jokes because they want that person telling the joke to like them whether as a friend or something else. It's perfectly natural to want to please people you are interested in, so this is most likely what that girl was trying to do.
But don't get your hopes up just yet. Some people laugh at lame jokes because they feel obligated to do that so they won't hurt the feelings of the person telling the joke. It's not just a matter of pleasing them because they're interested but more of a matter of them wanting to be nice. Sometimes I find myself in this position and I laugh anyway. Although I don't actually find it all that funny, I find it pathetic enough to manage an effortless, realistic sounding laugh.
And of course, there is the possibility that she actually thought it was funny and there was nothing else to it. People have different sense of humor. You may have thought that it was a stupid joke, but to her it could've been something totally hilarious. That does happen sometimes, so it may not necessarily mean that she likes you.
That's why, if you really want to make sure, then tell another lame joke. Better yet, tell at least three lame jokes (but not all at once of course). It's highly unlikely for her to laugh at several consecutive lame jokes unless she likes you (or has a very odd sense of humor). People who laugh out of sympathy would tire out at laughing much earlier than people who laugh because they like you. So by the time you make your fifth or tenth or whatever lame joke, those who laugh out of sympathy would have already dwindled away.
