Otaku, at least in the US, has a tendency to simply refer to anime and manga enthusiasts (at least, that's how I've seen it used). It's my understanding that, in Japan, it's a bit more similar to just calling someone a nerd, and literally means 'your house" in reference to rarely leaving theirs.
In any case, I didn't want to the last panel to be insulting, but I also needed it to be visually obvious that these people were into anime and similar media (if not for us, then for Catalina). I think I succeeded. That's a pretty rad hoodie right there, and those are some impressive feats of hairstyling right there.
As for why Catalina considers this an "eep" moment, she'll unleash her Catalina logic upon us on Friday.
Sidenote: I had originally planned to put some parody anime titles on the Otaku... Otakus? The plural of "otaku" shirts. I didn't get far with this before deciding it would be better not to have text on their shirts, but I was plotting such gems as "No Punch" and "Dragon Ball W".