Nina was hunting two Mega-sized monsters to bring back to Arad, which was an old tradition of the barbarians. When two are wed, the weaker one who lost must go out and hunt a large monster for their significant other to prove their worth. The bigger, stronger, and rarer the hunt is, the more impressed their partner should be.
To prove her worth to Arad and show her love, Nina had to find a monster that is so massive, unique, powerful, and most importantly, edible for Arad. Since Arad can eat pretty much anything, she didn't have that much to worry about, besides finding and catching her targets.
Thankfully, while her senses are absolutely overpowered in her human form, they are superior in her Tarrasque form. Merlin couldn't even study her body in that form due to how large she was.
