I learnt some French today (thanks, Twitter)

[Wildeblood]

Obama est un #bouffon.

Obama is a clown. This might be quite useful.

[Day]

Bouffon is not as neutral as clown. It's used by young (now older) inhabitants of the ghettos.
But you can use "clown" if you wish! Just pronounce it klewn :-p

[Wildeblood]

Google says it means "...is a joker", but that doesn't sound as natural as clown in Australian.

Have I misunderstood the usage? Is Obama plain dumb - a clown - or playing dumb - a joker - or something else?

[Day]

Well, in fact, it's an old word "buffoon", "joker", the role of the guy in stupid clothes making jokes for the king.

It has known a new youth recently due to it being used by youngsters from the ghettos, then went into local pop culture.

It's currently used in quite the same meaning as in Obama is a clown; derisive, derigatory, pitiable. You would not trust a "bouffon"*.

So, the usage is right, it's just it's only that 10% of the youngs use it, so 10% of 20% of the population, so 2% of the population using it, and then only when speaking.

I'm quite sure my parents would be baffled at this usage of the word

*: ps: ironically, people whose trust is worth something wouldn't trust people using this word.
Said another way, for 98% of french, only buffoons use the word buffoon.