What's the meaning? Like, they have a flat ass?

They're (jokingly) saying they believe the moon is flat. Like a flat earther, but the moon instead of the earth.

They believe the moon is a flat circle (not a sphere).

Which is perfectly ridiculous. It's obviously carrot-shaped!

Wait, are you saying that what we see is just the tip?

One half of carrot mooners believe it's the tip, the other half believe it's the butt. I'm a butt man myself

Who are you who are so wise in the ways of science?

It’s a truncated cone. It extends infinitely away from us but is aligned such our view is perfectly tangent with the sides, so it appears as only a circle to us.