It depends entirely on the cards and what's done when a card is missed. If the cards are straight definitions then probably not the best way to "get good at math". But if the cards (you can do this with plugins) are things like generated problems using specific mathematical techniques (solve a system of linear equations, for instance) and after failing a card you can go back and study the material, then it's effective. You can also throw textbook problems into cards if you don't want to use a generator, but that's a lot more effort than most people are willing to put in. You don't even need to put in the solutions if you're willing to keep the book around. It does mean cards take longer than a few seconds, but it still has the effect of taking advantage of spaced repetition to develop mastery of a technique.