You can success at a case interview even if you math is bad... but only if you are particularly lucky - and nobody gets lucky every single time. Consultancies will make sure to test your math skill in at least one of the interviews, if not more.
How to get better is simple however: practice. There are different ways to do so, including using one of the many specialized books or of course using PrepLounge's solid material. Personally, I used caseinterviewmath.com about 10 to 15 minutes of day for a couple of weeks; the site ranks you by accuracy and speed so you see where you stand and how much you progress.
PS: If you think you are truly bad with numbers, please don't even try to join one of the top consultancies - you'd waste everyone's time. I am sure there are highly specialized consulting shops who need more qualitative than quantitative skills, but I don't know who they are. All the largest shops will require a solid background / affinity with numbers, no two ways around this.