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It's no exaggeration to call her an angel.
Literal
She [topic-は] angel is [even if say-と言っても] exaggeration [not-ではない].
~と言っても過言ではない is a set rhetorical pattern: 'it would not be an exaggeration to say X.' 過言 ('overstatement/exaggeration') is a Sino-Japanese compound used almost exclusively in this fixed expression. The pattern is common in essays, speeches, and persuasive writing — it's a formal way to validate a strong claim while appearing modest.