¿Porque se dice "más ñoña" y no "muy ñoña"?

Hola Gary

This is how we express this type of exclamatory sentence for "What a [adjective]!" 

We use this structure in Spanish, as explained in the lesson that the exercise is tagging:

¡Qué + [noun] + más/tan + [adjective]!

You can of course say it without being so emphatic and just say:

Soy una persona muy ñoña. (I am a very nerdy person.)

but it wouldn't be highlighting so much that adjective. 

I hope this clarified it. 

Here's the lesson related to this structure. 

Saludos

Inma