I am much more familiar with math.SE. On that site there are explicit guidelines about LaTeX in titles and things like
\dbinom and similar are explicitly discouraged. See Guidelines for good use of $\LaTeX$ in question titles and perhaps also Why no use displaystyle in titles? on meta.math.SE.
The rationale behind this is that typically such title takes a lot of space in most places where the title is displayed. (Such as front page, list of related questions, search results.) If the same information can be displayed without the title of particular question taking much more space than others, it is probably an improvement.
This effect is more visible if you compare it with other titles not having this size, but even if you simply look at the search results of title:displaystyle or title:dfrac you can probably see what I mean. (There are also several relatively recent posts with titles containing displaystyle - from this month - so if you want to see how this looks in the list of recently active questions, you can simply choose one of tags in those questions and you don't have to go too much back to find such questions.)
However, MathOverflow users might have different opinions and local guidelines on titles might be different. So I wanted to ask on meta about this first. (I definitely would not like to impose standards of another site here on MathOverflow.)
I have also took arjafi's query from here and tried to modify it just to check whether in the past it was customary to edit \displaystyle or \dfrac away from the titles.
It seems that they were usually not edited. If I search for questions where displaystyle is contained in revision history of the title and questions with title containing displaystyle, I get the same results. If I try to do the same for dfrac here and here, I was able to find one question where I have replaced \dfrac by \frac.
However, take this data with a grain of salt, since I am not very well skilled in using SQL and SEDE.