The joke kinda got lost in translation but the idea is that Kagami looks like she's tied her hair in the way female kendo/kyudo/similar martial arts practicioners tie their hair before practice
Tsukasa, you look lovely in that ponytail... Why aren't you real, I go day after day after day after day after day after day longing for your love but it will never happen.
There are people in both animation and live-action who can look stupid in a ponytail. Kagami looks better with her hair untied. Tsukasa actually looks more like a man in a ponytail. Sorry, Konata, you are wrong.
@Jenthegerblesamurai I know there not, heck I like vocaloid as much as the next guy, erm it's not that obvious I have never heard the name Opal before... well exept raritys cat on MLP so I wouldn't know it's a girls name
Nothing wrong actually, they were just making a jab at the fact she looked really sharp like a Japanese swordsman. Probably had to do with the way she was posing too.