Applying to Math PhD programs with an math degree completed online
First, @scaaahu correctly observed some formal complications: e.g., looking at that web page, it seems that the closest thing to a degree in mathematics is a baccalaureate in "arts, letters, and science". It is important not to be even inadvertently inaccurate about your degree.

But, apart from that, the issue I'd anticipate in your situation is not the name of the degree, nor the on-line-ness. After all, one can read books and papers (or PDFs) without being registered at all. The problem arises in how to "prove" to an admissions committee that you have learned things.

In the U.S., even a transcript that says an undergrad got good grades in typical-sounding math coursework does not enable an admissions committee to understand what the student has learned. You should take the GRE (both general and subject test in math) to add data points and to show awareness, despite the fact that there is not universal agreement about the significance of these scores.

The problem is that you will not have letters of recommendation from three PhD'd mathematicians. Glowing letters from people who do not have PhD's in math (or something closely related) are not optimally helpful to admissions committees for predicting your success in grad school, much less gauging your natural talent. Very often these letters of recommendation are of decisive importance for admissions to top-50 programs, at least.