The doll should be able to stand on it's own- especially if it is so new. My Lissie is from 1991/1992 and she stands still.
If you have not played roughly with it, accidentally dropped it, or it fell off the shelf when a door slammed or someone stomped upstairs, than it seems like it is a manufactoring defect. If any of these things happened, then the strings probably loosened.

If you want her to stand, then you should get it fixed. If she mostly sits or you have doll stands, then you don't need to spend the money unless you want too.