Is it mandatory to apply for a 90-day extension in person after traveling outside Thailand?

Correct .

90 day extension? I think you must be talking about your 90 day report.

I think you're not talking about a 90 day "extension" at all 😮 and instead mean a 90 day report <- that's due 89 days from the date you stamp back in to thailand.

People have been denied online 90 day reports with the reason that the first 90 day report from returning from abroad has to be done in person, BUT other people reported they did one and were approved after returning to thailand from abroad

SO

you're out nothing by trying to file a 90 day report online, I mean the worst that will happen is they'll deny it and who knows maybe they'll approve it

Your 90 day report is due 89 days after you stamp in to thailand (the day you stamp in is counted as day one no matter what time you stamp in) so I'd try to file online 2 weeks before that date and see what happens