If kinetic energy is converted into thermal energy, so how hard do I have to slap a chicken to cook it?

Kinetic energy will also get converted into distortion energy and you won't want that happening with your chicken.

No matter how hard the slap is it won't create that much heat. Only a very small portion of kinetic energy turns into thermal if there's no significant friction to stop that motion. However you can create heat if you break a particle and Knock out an electron from it's orbit. Though it'll burn you also.

If you're using a mallett, the chicken would be pulverized before it reaches cooking temps. If you're using your palm, your bones would be broken before you get to that point.

Because so much energy is lost in the conversion, you cannot produce enough thermal energy to cook anything just enough to burn your hand

It isn't how hard, it is how many that us more important.