D.K.
Both of my kids did this at one point in diaper days. Just explain to her all the germs in poopy and how it can make her VERY sick if she got any in her mouth. Try zipper feety jammies and then safety fasten it at the top, that way she cannot get it off or undone. It is normal for their fascination as it is has the consistency of playdough, however you need to drive the point home it is yucky to play with. Don't overdo the yuck factor as that can cause issues with potty training however with girls I found it easier to get the yuck factor across to them. Be stern if you catch her and tell her that is a big no no....
It will pass, just keep it out of her mouth as you can get a nasty stomach upset from it!