"Is that a kiss" is a literal translation of a question that has no meaning in the given context beyond being a loathesome pun on Rumia's fanon catchphrase. "Kiss that so" correctly expresses the intended meme and is therefore the better choice.
Eeek?!*Smooooch*Mmwuh?!......Mmwuuh!!Isn't it hotter if you stick so close to me?The heck is this?!Well, I feel cool now so it's fine.Make me feel hooootter...*Chest grope*Kiss that so~*Rustle*It's hoooooot.*Grab**Zzoom!*Wait, what? What's this!?
Rumia?