Greenhorn is an old time term and NOT mean.

A beginner who made cameos was a novice who made mistakes because they made a bad call. They might make a very fine cameo but it turn green and so nobody would buy it.

Everbody when they start out is a greenhorn. It's akin to being an apprentice.

Every mentor started out as an earnest apprentice then would make a mistake and be teased and called a greenhorn.

It is not unkind at all, but affectionate as mentors remember the goofy mistakes they themselves made. Like picking persimmons too early with insufficent frost and so the persimmon is so astringent it's inedible. You went to all that trouble knocking them down and making a pole basket and then it was a waste of time.