hey unagitsukino
first of, the divot lines you can see: these are caused by the normal map not your diffuse. you haven't made a new normal map for your new texture, so the game uses the normal map of the skirt you cloned to make your .package. to get rid of the lines you can either make your own normal map, e.g. by following
this tutorial or you can replace the normal map in your .package file with a blank normal map.
as far as i can tell you also haven't changed the shadow and specular maps of your .package. to avoid further issues you should also either make custom versions of these or replace them with blanks.
now on to the funky waistband of the skirt! it is weird because the top row of faces of your mesh rise above the waistline of the sim where EA end bottoms and begin tops. so basically the skirt is too tall and intersecting with geometry above it. it is possible to make skirts and pants that have meshes reaching up into the part of a sim reserved for tops but i haven't tried to make one so i can only give you some very general tips on how to do this.
what i would do is grab an ea skirt mesh and take a look at where it ends at the top. then i would shorten your mesh by moving the vertices at the top of the skirt to align their height on the z-axis with the vertices in the EA mesh. you should also make sure that the top row of your vertices lines up exactly with their counterparts on the EA mesh.
if you want the texture of your skirt to go higher up the sims' body than the EA waistline you just draw up into the top area of the diffuse map until you like the result. to do this you don't need any part of your mesh to be above the waistline, since any top should provide the necessary geometry. if you want a uv guide for the sims uv map outside your mesh you can clone the nude top in studio and export it's uv map via blender. together with the uv map of you skirt this should cover the whole sim without overlapping anywhere.
however doing high waisted textures this way can lead to issues with intersecting textures with some sims' tops.
if you want the skirt's mesh to extend into the top area i would edit the waistline so that it is slighly outside and away from the nude top mesh in the area and make sure that this distance is even all the way around the waistline. currently some parts of your waistline are a good distance away from the nude top but others are very close or even inside the nude top like the center front of your skirt. only removing this intersection by moving the affected vertices outwards should do a lot to make the waistline look better even if you don't do anything else.
to make a uv map for this kind of mesh you should make sure that none of the skirt's uv map extends up into the area of EAs uv maps that is reserved for tops (you can use
this as a guide). this method can also lead to intersection issues, they're just different from the ones caused by the first method.
your uv_1 could also do with some work since the top ruffle at the front of the skirt is still positioned in the same place as the top ruffle at the back of the skirt. i assume you duplicated the back to make the front? if there aren't any issues with gaps or morphing when you increase a sims weight or when they move then you could leave it like it is but i would still recommend following a uv_1 mapping tutorial to improve this map by moving everything in the correct place.
don't forget to make lods1-3 for your final version once you are happy with your lod0 mesh!
i hope this helps you out!