I reported the exact same issue here the other week, with no replies. Other flavours of Ubuntu 21.04 run fine.
EDIT: Interesting... just reflashed a USB, and MATE 21.04 works fine now. Only difference in my system is that the attached HDMI monitor is 4k (2160p), not 1440p. At a guess, something to do with Ubuntu's default switch to Wayland doing something with the display that X doesn't like? Fractional DPI scaling?