To Pai : run Blender, import your skirt tmcmesh + body tmcmesh, clic on the skirt and go in edit mode at the bottom then move the skirt how you want with the colored arrows 
When you are done, go back in object mode and export your tmcmesh then add it with Polygon Tool (dont forget to remove the skirt mesh on your tmc before add the new one).
To Coopermon : if nothing changed after swap a costume then you should try in an other .bin, some costumes are in 2 .bin and only one work.
Other solution : you can still swap on an other costume if you dont find how fix it.
When you are done, go back in object mode and export your tmcmesh then add it with Polygon Tool (dont forget to remove the skirt mesh on your tmc before add the new one).
To Coopermon : if nothing changed after swap a costume then you should try in an other .bin, some costumes are in 2 .bin and only one work.
Other solution : you can still swap on an other costume if you dont find how fix it.