The general culture of the site, relative to other sites, is that rather little editing is done, and that people write as they see fit. There were some run-ins over edits of the type you mention in the past. Yet, some editing is still appreciated.
There is a recent moderator post on the general editing etiquette Editing etiquette
The edits you describe are covered by "Embedding or re-uploading images, fixing formatting, etc."
To answer you specific questions:
Is using MathJax for formatting discouraged at MathOverflow?
To say it is discouraged at MO might be slightly too strong, but at least using Markdown for this type of formatting is encouraged.
A somewhat related discussion on On using MathJax to create 'white space' could give an idea of existing points of views.
Is it acceptable to use MathJax to achieve bold or italicized text in titles, where MarkDown is not available?
It might be acceptable but not few will consider it as quite unnecessary in almost all cases, and even the use of MathJax for formatting (simple) math in titles is not universally approved.
See the discussion at Editing titles to include $\mathfrak{LaTeX \; Math}$ (the original subject is somewhat different).
One should also keep in mind that titles specifically are also displayed, or at least could be displayed, at places where there is no rendering available (e.g., some ways to access MO on mobile devices will not render titles). Giving another reason to minimize use of MathJax in titles, especially for the purpose of formatting.
My specific recommendations are:
If you notice a new user using MathJax formatting to bold text and alike I think editing it with an explanation about Markdown is a good way to proceed. It is a lot more likely they did so as they thought it is the way to do it, or at least did not know another way, rather than deciding to do it in this way knowing both.
Put differently they just formatted the post as they would in LaTeX (more or less), which they know. Sometimes one sees (non-working) \emph{some text} or LaTeX-style diacritics like "Pr\"ufer" on the site too.
For experienced users, especially if they use it a lot, I would be more careful. In case you want to avoid risk of confrontation, leave a comment, but better do not touch the post. (See the starting paragraph.) Also, try to avoid to comment the same thing on multiple posts of the same user.
For titles, I would recommend about the same but would encourage more editing for the reasons explained.
My very personal view would be to encourage rather more editing than I did above, but as a general recommendation I prefer to be more restraint.
\boldsymbol
as in $\boldsymbol{boldface\ italics}$. $\endgroup$\textsf
will be added to the standard MathJax commands, to slightly improve in-math text formatting. And that was my point, that while MathJax supports\textbf,\textrm,\textsf,\textit,\texttt
, it does not support any mixing of them while retaining "text mode spacing". $\endgroup$\textit
when, for example, introducing terminology; the reason is that I find markdown italics not standing out sufficiently. I am happy with markdown bold though. I think if text versions of fonts (with proper spacing) are used, then it is perfectly OK. $\endgroup$***
, of course. $\endgroup$****
****means****. $\endgroup$