- Tanks are normally associated with exaggeration. When you see someone driving a tank (mainly someone that normally doesn't), you think that's funny because it's very exaggerated.
- Cartoons are generally associated with comedy. Animes are cartoons, so it's easy to make that association from the get-go.
- It's about girls who are; on a debatable nature, beautiful. When you see a beautiful person,
you tend to get red and start laughing, which is also a common reaction to something funny. - It's common for people and events of World War II to be parodied on films, cartoons, games and other media, so it's easy to see comedic material in the subject.
- By making important figures of the war be replaced with cute girls, the writers have an opportunity to play with some silly and comedic twists, that both subvert war movie tropes and anime tropes.
- Going back to the first point, making cute anime girls drive tanks is rather comedic, because those girls look kind and sweet, but they are driving dangerous and destructive vehicles of death.
- Its something easy to meme about, and, well, memes are supposed to be funny, so, if it's all going right, you should find it funny too.
- Not sure if any of you noticed, but an army marching for a battle can be a rather funny situation, depending on how it's done. The music, camera work and character expression needs to help the comedy, and I assume that's what the writers intend to do.
- Again, it's about cute little girls, those normally aren't soldiers, or generals, or have any involvement at war. It's a funny concept by itself.
- It's just funny, it's the funniest.
So yeah, the connection of getting shy and chuckling because of a beautiful girl and laughing because of something funny is easy to see.