Is poor English grammar and writing style overlooked by peer reviewers and editors in otherwise strong math paper submissions?
Short answer: no
Longer answer: The purpose of the article is to communicate something. If poor grammar and style gets in the way of that goal, then that is something that needs to be addressed before the article can be published. If the paper is indeed strong, then that will typically mean you will get a "revise and resubmit" rather than an outright "reject".
Sometimes (as a referee for a submitted paper) I have provided detailed grammar corrections1. Other times I have just said: "The English is unacceptable, get someone more expert in English to correct it for you." I do seem to recall in one case the editor told me that the journal staff will correct the English, so I should not worry about it, instead only worry about the technical content.
1 éventuel does not mean eventually
Are you asking about the "should" or the "is"? It depends on the referee and the editors. I have seen papers published in top venues that would have seriously profited from 5 minutes of proofreading (sometimes even from an automated spellcheck). But I have also seen referees point out rather subtle linguistic errors carefully and in detail (and have done so myself a few times). There is probably no shortage of referees who are willing to rubber-stamp a paper written by a celebrity no matter whether any of it makes sense. I also wouldn't be surprised if an editor feels pressured to publish a paper by a VIP even despite the objections of referees.
There is also a huge distinction between the kind of errors that merely reveal the author to be a foreign speaker, and the kind that make reading difficult. On occasion, a language barrier completely prevents the author from readably communicating a proof; while I cannot blame the author for the "mistake" of being born in the wrong country, I don't hesitate to send back such a paper for revision, after giving as many suggestions as I can (and as concrete as I can) for how the writing could be improved and what pieces of syntax they seem to be missing. Things like this should not lead to rejection of papers, unless the author fails to clarify their work through several revisions; but the paper should not get accepted until this work has been done.
It may or may not be so with the referee, but it is rarely overlooked by the readers.
Few researchers have the luxury of spending time deciphering bad grammar and convoluted sentence structures: battling typos is more than enough. Moreover, if the community is left with the impression that you write poorly, they will shun your work unless it is brilliant.
Too many spend months working on a project but limited time thinking about presenting the results in a reader-friendly manner. Good grammar, the systematic use of a spell-checker, and adequate clarity result in a win for the authors and the readers.
This question really makes me remember a good editor, who gave me an opportunity to revise my paper even if the paper was really written by very poor English. That was two years ago, when I submitted my first paper, I got rejections almost from everywhere because of the poor paper writing. Finally, the paper was publicized and now I am a second-year PhD student with 6 papers in hand. They are not top journals but they are all in Q1. All professors grow up from young students. I believe everyone produced bad papers when they were young. It is true that the paper must be publicized in a good format otherwise readers will get confused when reading papers. However, the editor could do more than a rejection. If the paper really has some good things, such as good idea or math, maybe one more opportunity from the editor will give the world one more good professor. Thanks to the editor.