So my question is, has the D.A. degree in math been taken seriously?
How seriously the degree is taken is more likely to be influenced by which university granted it than the name of the actual degree. This is especially true for uncommon degrees like a D.A. in Math
I'm going to assume you're considering a D.A. in math and are asking to find programs you'd be interested in.
I would not put very much stock in the name of the degree for the following reasons
1) The name and the umbrella college each school inhabits in a university is usually influenced by politics.
The likely scenario is when the school was created, there was some in-fighting to get the shiny new building and the new resources that school would bring. For some reason, it ended up in the Arts department. For example, the MIT Media Lab is part of the Department of Architecture, even though it has little if anything to do with it.
2) Even if the program was inspired by this book, the founding professors have likely retired or are near retirement due to age.
It's a pretty old book, and the idea hasn't seemed to gain much traction. Even if the programs were originally designed to focus on teaching, new faculty have likely been hired and put their spin on it. I highly doubt any program at a university shares that much with it's 40 (or more) year old counterpart.
3) Some professors research effective teaching methods in their own departments whatever the degree is called
Schools such as Rose-Hulman are almost completely teaching focused and award tenure based on excellence in teaching.