No, I don't think it's necessary to use MathJax. In fact, one of our top answerers here doesn't use MathJax at all. This is mostly because answers, and questions, are based in physics and not in maths (though often the maths & physics are intertwined so it's kinda difficult to distinguish them). So if you can explain your question, no matter the subject, without maths, that's perfectly fine.
That said, it's useful to write your equations in MathJax because it helps readability for more complex statements. Obviously writing $\psi$ versus ψ will result in anyone picking up, but if you've got an equation or an integral, it's going to be harder to follow what's being asked without it written up in MathJax.
I've actually seen a case where someone wrote something along the lines of "integral of f(x) from 0 to infinity" in the question because they didn't know how to write it as,
$$\int_0^\infty f(x)\,\mathrm dx.$$
The point taken is the same in both, but it helps readers read more quickly.1
Hence, you ought to write your mathy things with MathJax.
Note that, in many cases, the users here are friendly enough and help fix MathJax issues, so even if you don't get it quite right, someone will help you. And if don't know the notation fully, there's the ever useful Detexify which analyzes your input to find the matching LaTeX/MathJax command for it (I've not tried it on iOS, so I can't speak to how good it is there, but on Android & desktops & tablets, it's just fine).
1. Not the case I was thinking of when writing this, but it just happened today again: https://physics.stackexchange.com/posts/464590/revisions