Does doing an undergraduate math major make an applicant for philosophy phd programs more competitive?
Recommend getting the math major if you can. (I have double bachelor's in philosophy &amp; math myself, although my graduate school and professional work was in math.) 

The undergraduate study is the best time to expand yourself into as many fields as you'd care to take. Generally, graduate school is the place that demands exclusive focus on one narrow area. So the opportunity you have now will not be available later on; and the degree will document permanently that you can achieve in any field you need to work in. 

Plus, there's the possibility that later in life you may wish to adjust or switch fields. Having the math degree will allow you to immediately step into programs in any science-related area you wish. 

Standing astride two separate fields like an academic Colossus gives you a perspective and options that few others can match, and now is the very best time to establish that.