Decided to translate it like that because it seems to be a slight pun on Kyon's phrase "ponite~ru ga moe da", whereas here she says "ponite~ru moe nan da".

Ponytails really turn me on

Stupid sexy Kyon...

Yes, Ponytails are sexy.

Or Kyonko could've been saying; "What is a 'moe ponytail?'"

I don't know if she would've lost her knowledge of anything otaku, being genderbent.

Being that there is a heart there, she could also be asking in a loving, playful manner?

But going along with the heart after the dialogue, what you've 'translated' it as.. does indeed prove what is said.

Ponytails ARE sexy.