can you also say ona není špatná?
but ona neni spatne was wrong...
špatně is an adverb, not adjective
Why "she" is not translating?
Because we often skip the personal pronouns in Czech where they are not necessary. The grammatical form "špatná" clearly determines it is a woman or something grammatically feminine.
When is it appropriate to use "neni" versus "nejse"?
není is the only irregular negated verb form in Czech, used instead of the excpected neje. (btw 'nejse' doesn't exist in Cezch)