Formal:
* C'est moi qui suis le dindon de la farce. (This is the most literal translation, meaning "I am the turkey of the farce".)
* Je suis le bouffon de la situation. (This translates to "I am the buffoon of the situation".)
* Je suis la risée du monde. (This means "I am the laughingstock of the world".)
Informal:
* Je me suis fait avoir. (This translates to "I got caught" or "I got fooled".)
* J'ai fait le con. (This is a very informal and somewhat vulgar way to say "I made a fool of myself".)
* C'est moi qui suis le piéton. (This is a playful way to say "I'm the sucker".)
Other options:
* Je suis dans le pétrin. (This means "I'm in trouble" and can be used when the joke is on you in a negative way.)
* J'ai commis une erreur. (This translates to "I made a mistake" and can be used when you're acknowledging your own fault.)
The best way to say "the joke is on me" in French depends on the specific context and the tone you want to convey.