To be fair to her, she's not unattractive.

Literal

Her fairly evaluate [if-ば], she [topic-は] homely [copula-ではない].

公平に評価すれば means 'if you evaluate fairly.' 不器量ではない means 'not unattractive' — 不器量 is a somewhat old-fashioned word for 'plain-looking.' A carefully balanced assessment.