Nothing fancy. Brown bread, vegetables if your lucky meat... oh and lot's of herbs. Sometimes you just have a stew. If you have a bird like a
Falcon you would have your falcon hunt meat for you.
Court jesters worked for lords, and were referred to as "licenced fools." The term licence here did not mean they needed to obtain a licence from some agency of the government or guild. This word is related to the word licentious - they were allowed to do
anything, as long as it was funny or instructive. That might include grabbing the lord's dinner and chowing down on it, it if was funny or instructive.
The things they did were relative to the court they operated in. Their comedy was not like stand-up comic acts of today. It was personal and relevant to the court or household. For example, King James VI of Scotland had a jester named George Buchanan. When George noticed that the king signed documents without reading them, he acted in a way that was comic, but it was also instructive to the king. He slipped a document into the king's daily work that proclaimed the king was abdicating all royal authority to one George Buchanan for a period of two weeks. The king signed it. We have no record, that I know, of saying what George Buchanan ate for dinner that evening, but we know he was not fired.
In order to perform his duties, the jester had to have constant access to the court, and had to be able to act spontaneously. I suppose there were lords who had their jesters eat with the musicians and other entertainers, but that was not proper use of their abilities. Best use of a jester meant they should be present at everything except when state secrets were involved or privacy was important. So best use of a jester meant the jester ate with the lord and ate pretty much the same food the lord ate.