The correct translation for "My skirt is plain" in Spanish is "Mi falda es lisa."
The other sentences you provided are:
- "Mi falda es nueva." (My skirt is new.)
- "Mi falda es calor." (This translates to "My skirt is heat," which doesn't make sense in this context. It should probably be "Mi falda es caliente," if you meant to say "My skirt is hot" in terms of temperature or fashion.)
If you need anything else, feel free to ask!