As long as you have the energy to do so, please continue to correct such edits/titles.
Unless you have reason not to, assume the editors are well-meaning and just don't know that
\displaystyle should generally not be used in titles, and address a comment to them - editors of a question can be
@-pinged, although their username does not appear in the auto-complete list unless they commented on the post - asking them to stop adding it to titles. Linking to the meta posts about that issue in the comment is a good idea. If these requests don't lead to a change in behaviour, there comes the time to raise a flag describing the problem (if you happen to have a list of incidents handy to include in the flag message, that would be great; if not, the moderators will look for themselves).
A general remark about editing (not addressed at the OP of this question):
Whatever common mistake one fixes in posts, be that retagging wrongly tagged questions, fixing spelling mistakes, eliminating
\displaystyle from titles or something else, please don't do that in rapid enough succession to flood the front page with such edits. Take your time - and look over the post whether there is more in need of improvement.
(It frequently happens that somebody compiles a list of posts with a specific mistake to fix and then edits them in quick succession, without improving anything else. That is suboptimal.)