I'm guessing it's probably a comment to someone who has been telling jokes all day or something along those lines. It sounds like kind of a grandmotherly thing to say - but hey, I'm not really up on the Jugend-Slang these days, who knows. Maybe something like "Oh, we've got our clown hat on today, do we?"

That's correct. If, say, someone who usually is more of a serious kind continues on cracking jokes, others might assume that he must have "had a clown for breakfast" as (s)he wouldn't act like this under normal circumstances.

this is a friendly way of saying "you annoy me with your jokes today!" to someone who usually doesn't :-)

Kalauer:

Zwei Kannibalen essen einen Clown. Sagt der eine zum anderen: "Der schmeckt aber komisch..."