There's a common saying in French (at least in France), which says La culture c'est comme la confiture, moins on en a, plus on l'étale.
The pun is on étaler, which means both to show off (culture) and to spread (jam).
Hi, This is a very metaphorical sentence. I don’t know if there is an English equivalent. It means more or less “showing his/her incompetence as one spread jam out”, referring to someone who shows his incompetence and in such a way that it becomes clear that this incompetence covers many/everything (as jam does on bread). I hope this helps Cheers, Julien