Will bad grades in non-math classes affect admission?
I'm not in your discipline, so I'm not sure how much this is similar...When we evaluate doctoral and master-level students for admission, we first check if their GPA meets the minimum criteria. If it does, then we look to see how well they did in courses that would help them succeed in our graduate programs. If your GPA hasn't suffered too badly because of the non-math courses, they might not hurt you.  However, I would caution not to mention to admissions personnel/reviewers that you believe the non-math courses were "useless" so you blew them off. That doesn't sound very professional- if asked, I would say that you focused more on the math courses to prepare for your graduate studies, which took time away from your other coursework.