prof-peach answered:
Hey, though I’m not a ghost type expert, I do know a bit of textile knowledge, and sewing your friend a new cover is very plausible and fun thing to do! Some people give their mimikyu a whole wardrobe of outfits to choose from, others pick materials that are shock proof, fire resistant or fantastic for resisting tears and puncture wounds.
Polyester is a very good option. It’s widley available, fire resistant to a degree, and quite easy to work with. Sewing a new cover for your Pokémon from this material will leave them with fair speed and good defence. Another option is durable canvas. It may slow your partner, but increase their defence. The weave in that material makes it harder to cut through and pierce in battle.
Cotton is breathable and somewhat strong, stronger than the rags mimikyu usually use for covers. There’s also options embedded with oil based products that are waterproof, and sometimes comparable to tent material. I would strongly suggest taking your little friend to a textiles shop, get them to feel the materials, see how they bend and move, and let them choose one they prefer, or one suited to the battles they get into. If you feel really fancy you could make a silk cover, a strong material, that moves very well if you compete in contests or just like to look your best.
Your options are pretty endless, but be aware that some materials are easy to tear and break due to the way they are put together. A rule of thumb is to look for materials with a twill weave. It gives you strength, lightness and flexiblility.
I hope this helps you choose, though it truly is down to you and your partners preference! Good luck out there.