I tried the 20.04.1 armf version on a Pi3. I got no usb, so I could not complete the installation. After lots of fiddling i concluded the armf binary was defective. Same car, same hardwar, installed the arm64. usb woks and I am merrily on my way. Files were ubuntu-mate-20.04.1-desktop-arm64+raspi.img and
ubuntu-mate-20.04.1-desktop-armhf+raspi.img. A key is to look at the mousewhen you boot. If the light is on keep going; if not try something else. Hope thgis is helpful to someone.
Thank you for your report! I will try to get this to the right place.