Yep, wha;t happening is that, in order to confomr a skirt (or apron, in this case) you have to have a set of polys grouped to the hip between the thigh groups (or you will get tearing). So the thigh groups bend the confomer to match the bent thighs of your character, but the hip group remains straight horizontal matching the hip group of your character. This is one of the biggest problems with confomring clothing at this stage of the game.

There's not a lot that you can do about it EASILY. You can use the morph brush to try and move the un-bent polys into place (which will create a morph that can be saved for future use) but takes some skill, practise and patients. You can export the apron using a program like Poser morph Loader and make your morph in an external modelling app. You can also try and use a magnet to bend the polys into place. If you feel compfortable you could exprot the apron as a single group object, import it as a prop and use the cloth room to make the apron into a dynamic clothing.

The magnet is probably the easiest options. Assign the magnet to the hip group in the apron ONLY, and scale it so that is only affects the polys you want moved and than move them into a position you like. If this is a one shoot for a single render, then you are done. If you may want to use this pose again then spawn off a morph once you have the shape you want.