"She is nice."
Translation:Elle est sympa.
"C'est" is, I believe, only used for modified nouns, not when adjectives are used by themselves. "Jolie" (and "Joli") means nice in the sense that something looks nice, so if the woman is "jolie", I'd say a more natural-sounding translation (to me at least) is "she is pretty".
Or to put both those things in a sentence, "C'est une jolie jupe" = "It's a nice(-looking) skirt".
I learned this the hard way, by losing hearts!! "Elle est" and "il est" become "c'est" only when the next word is an article (un, une, le, la, or l') or a possessive pronoun, e.g., mon/ma. So, "elle est gentille" but "c'est mon amie". The last tidbit includes a bonus: the masculine "mon" is used for feminine nouns starting with a vowel. ;-)