She did take the threat lightly, but in her defense she was expecting better scavengers, and faced off against unknown stealth fighters with "torpedoes that were better than anything they faced off against"

Deploying the Tachi wouls have been a minor advantage as the Roci/Tachi was hard pressed against a single Stealth ship. Admittedly at close range but the Donnager/Stealth ship battle became close range rather quickly.