Honestly there's not much to this theory other than he's like pushing 30 atp and still cosplaying Kamina from Guren Lagaan, who is so clearly this character he idolizes as the ultimate brother figure that inspired his little brother, which is exactly what happened to him and his alternate universe Dave.
Guren Lagaan has a lot of corny themes that are rooted in optimistic philosophy, and Dirk's very much a nihilist who believes in the practical and won't even entertain the very idea of maybe being happy with the way things are.
The Kamina cosplay is his "Ben Stiller shades"; Dave's shades are important to him not just because he thinks they look cool but because they came from his best friend, whom he admires a lot. It's an important facet of his identity because it's tied to one of the few relationships he has with someone else that is important to him.
But he is too egotistical to admit this, and thus has to make it a huge ironic thing when, in reality, it's clear he probably watched Guren Lagaan and just thought: "ugh this is lame and cheesy but it does inspire me a lot..."
Anyways, I don't think he's a full on weeaboo or Otaku lol. I think he just enjoyed Guren Lagaan more than he wants to admit.
There's also something worth being said about the fact that he definitely watched Guren Lagaan and got far enough into the anime to cringe at the "Don't believe in me, believe in the me that believes in you!" line when Dave and Karkat confess. And that he chooses to dress as Kamina instead of Simon, the actual main character. Dirk doesn't have a Kamina in real life and can only idolize his bro from afar since he and alt!Dave never actually knew each other. So I can see him just deciding to go "Fuck it, I'LL be Kamina. I don't need someone around to inspire me, I can do it myself!" and literally dressing up as the show's single embodiment of hope.
It's definitely silly but I like this theory a lot. It is really funny to imagine a 30 year old guy still permanently cosplaying an anime character he refuses to say he likes.
That's it, I'm gonna do what I should have done a long time ago.
I'm going to beat him to death with this hammer.
That image is so fucking funny I am definitely borrowing it to spam at people