I think the idea is actually pretty good. It mixes quick math with a bit of puzzle solving, and the timer probably adds a nice bit of pressure and makes it more engaging. The monster theme sounds fun too and could make it feel less like “doing math.”
Having multiple levels and difficulty settings is a smart move since it can appeal to both kids and adults. My only thought is to make sure the early levels ease players in before the timer gets stressful. Does the difficulty increase by giving bigger numbers, more rows, or less time?