I can't correctly view formulas in comments on SE app for iOS. I've an iPhone 6 with iOS 10.3.3.
There is an indirect way. If you tap the comment, a little white bar should roll out underneath, which has a button titled "$\color{red}{\cdots \text{more}}$". Hit this button and then hit the "$\color{blue}{\text{Render Mathjax}}$" button. A new screen should open with the Mathjax rendered.
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2$\begingroup$ The same kind of menu selection is necessary on the Android SE App, in order to render $\LaTeX$ in Comments. $\endgroup$ – hardmath Jul 23 '17 at 22:22