Did you figure out where you wanted your cutie marks?
Cutie marks would have slightly different placement on different types of clothing: ruffled skirt vs. jean skirt, pants vs. shirt. I say anything goes as long as A) it is visible and B) the location makes sense based on the type of clothing.
If you have a straight skirt you could put one in front and one in back along the hem. If you have a ruffled/flowy skirt you might need to put it on the waistband or belt for it to show up. Hard to say without knowing what your skirt looks like. Also, looking at fanart might help you get some ideas for cutie mark placement.