Technically you can downvote for whatever reason you want. (Any reason based on the content of the post, that is. If you downvote because of who posted it, that's not okay.) So if you want to downvote a post because it doesn't use MathJax for the formulas, that is, strictly speaking, okay. It may or may not be a good idea, however.
I think there is sometimes an argument in favor of doing this. The main reason you're supposed to downvote an answer is that it's not useful. But an answer can be not useful for a variety of reasons: it's wrong, it's unclear, it doesn't justify its conclusions, or it's just difficult to read. (Or others.) And an answer with a lot of math which is formatted using plain text, not MathJax, qualifies as "difficult to read" as far as I'm concerned.
In practice, of course, it's an easy edit to put in the proper MathJax formatting. So when I come across an answer of this type, I'll usually just edit it, rather than downvoting. And in general, whenever the reason you would downvote is something that can be fixed in a fairly obvious way by an edit which doesn't change the meaning of the post, it's preferable to edit rather than downvoting.