It depends what you mean by your mathematical ability being poor. If you mean poor as in "under-developed" as you haven't done enough courses in it, then fine, do a Masters. If you mean that you are just a person who isn't strong at all in numbers, don't waste your time and money. Firstly, your course will be a living hell, and secondly, you'll get stumped anyway when you have to do the numerical tests at every single consultancy.