[his/her] mastery of calculus is limited to straight lines.
Seriously, it is not the best insult in the world, but it is certainly not used often and that definitely counts for something. What the insult implies is that the person in question is not very good at math so much so that basic calculus is beyond them. I know most people probably think calculus is hard, but I assure you, those same people use it without realizing it. To say that someone claiming to be a mathematician cannot grasp it is like saying a basket ball player does not know how to dribble (god I hope that is spelled right) across the court.