。
She is utterly oversensitive.
Literal
[topic-は] completely nervous-overly-sensitive [formal-copula-である].
神経過敏 ('nervous oversensitivity, neuroticism') is a Sino-Japanese compound with 神経 ('nerve') + 過敏 ('over-sensitivity'). It describes the clinical or quasi-clinical state of being easily triggered by stimuli — physical, emotional, or mental. まったく (often written 全く) is an intensifying adverb meaning 'completely, utterly' — paired with negative predicates it shifts to 'not at all,' but with affirmative it intensifies. である is the formal copula, here giving the sentence a clinical/literary weight; the conversational equivalent would be だ. The whole sentence reads as a diagnostic-style observation rather than casual chat.