Does Tempest Trials cumulative score max out?

yes! you can score much much higher than that. I believe last TT, the cutoff for top 1000 was around 132000 points. top 5000 was around 104000.

As far as i know it doesn't. I went well over 100k during the first one and there was no max score then. Plus with feather rewards based on ranking it wouldn't really make any sense for them to have a max score cap.