In the original Japanese, there are all sorts of puns to be made off that title. Very clever. No way to translate that into English, I'm afraid.
Aside: if Cirno thinks you're an idiot, you might be a member of Team ➈.
Final Note: I had to try so hard to resist the temptation to translate Cirno's "Atai-tachi" as "Neyether of us".
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You're on our side no matter how you look at it!I did, why?N--no way...Whaa!?That's all right. I don't gain weight no matter how much I eat. ♪Uhm...... but, well......?heeheeheewon't you get fat?Uwehhh!?WHA--WHA-WHA-WHA-WHA... STOP TELLING LIES!If all you do is eat anytime and anywhereHey!! What kind of understanding did you arrive at just now!?TH-THINGS LIKE THAT ARE IMPOSSIBLE!You're an idiot......Make the World JealousYou guys ought to say something to this insolent jerk too!!Cirno-chan, are you betraying me!?*boing!*pixiv pediaEven for us, if anything......By any chance, did you climb the mountain just to gorge on rice balls?*hanging straight down*Awkward!Ah.....Is that so......?
Although the phrase is identical to Rumia's usual question, in this context, the meaning is closer to "I see, so that's how it is."