For the T shirt
Shape : General overall shape are fine. However, it seemed to be a bit too straight on the chest. Lacking of wrinkles/creases. When you follow the breast shape that closely, it always suggest tightness, and when there is tightness, there should be more creases. The letter M seemed to be too flat, it needs to follow the curve of the cloth which hangs on the body.
Shade : You have got the lighting and shadow direction correct, bravo. But the shape of shading is too stiff and flat. The body is not flat, there should be some curve to it. When you add in more creases, you would get more natural shading. The placement of your highlight could be off a bit. Try studying the basic tshirt on menewsha and note how they shade the shirts.

For the Skirt
Shape : Very good attempt on the folds. However, the centre curve down position of the skirt was wrong. The avatar is a quarter view, so the centre is NOT the centre of the avatar, it is 3/4 of the avatar. You have done fine with the belt of the skirt of how it curve down, but it seemed to droop down very low at the centre. You have to look into pixel tutorial which explain about how to make curve shape with pixels, there is a certain number in play on making nice curve. The shape of each fold could also use some improvement when you master the curve and let the fold follow it. Currently, it just seemed like there is something weighing down the centre portion.
Shade : I think your shading need to change. I think I know what you try to shade with showing some under neath skirt, but it doesn't work out. You seemed to have a bit of pillow shading tendency at each fold. Again, your shading seemed to be very... stiff, it doesn't express the curve/softness/flowyness of the material.

Conclusion: there is a LOT to swallow. DON'T DESPAIR. Hit them down slowly one by one.
First off, for shape, it is better IF you have some kind of a reference. Fundamentally, it is better if you understand how the material characteristics are.
Second, google some tutorial about pixel shading.

No fret, you do have some good points. You manage to catch the light and shadow direction well. You just have to think and imagine the material you are pixeling.