"She's everything. He's Just Ken."

Barbie = { Everything }

Ken ∈ Everything

∴ Barbie is Ken

Given that Barbie is everything and that Ken is a member of the set Everything, we therefore know that Barbie is Ken.

@LilahTovMoon I prefer the french poster

@Billboquet For those who haven't seen it, the French poster says "Elle peut tout faire. Lui, c’est juste Ken."

"c'est" sounds like "sait" (he knows) and "ken" is slang for "f*ck". So he just knows how to f*ck.