Why are stepmothers always described as wicked?

Kamala would make a great wicked stepmother.

This has nothing to do with current-day politics.

The "evil stepmother" trope is a very old one that dates back to the earliest days of storytelling. It's a quick and easy way to have a villain in a story, or at least a quickly made antagonist to the main character. This also relates to the "dead mother" trope, because it was very common in times past for many moms to die in childbirth and the dad would remarry so his kids would have a mother figure of sorts to take on the role. Trouble was, the dad didn't always use the best judgement when finding a new wife.

The truth is, not all stepmothers are evil, and in some cases, the kid might end up loving their stepmom more than their biological one, particularly if the original is still alive!