Should a step mom be called mom?
I am going to say generally, no. It's one thing if you have raised the child their entire life as the main or only female figure. But if they have a relationship with their mother then it needs to be respected, in my opinion. That includes the title of "mom".
Now if the child wants to call you mom and you are comfortable then go for it. Otherwise it would be a better idea to be on a first name basis.
But my sister has 2 step kids who call her mom and their birth mom by her first name. But that is what their birth mom wants.