I'm not sure how many other Ubuntu MATE users also use QGIS but if you upgraded from Focal to Jammy and you find you have trouble reading either your menus in QGIS or the tool tips when you hover your mouse over an icon like I did, then here's what fixed the problem for me: I went into 'Setting', 'Options', 'General', (in QGIS), and then look on the right-hand pane for 'Style (QGIS Restart required)' , and set the spinbox over in the right from 'Fusion' to 'gtk2', and restart QGIS.
At least that's what worked in the end, all the things I tried in between are a long story. I hope this helps someone. The QGIS documentation is very brief when it comes to explaining this setting and this is the first time I have ever been faced with this problem after years of using QGIS in Ubuntu MATE. Normally new versions of QGIS just adopt the same settings we were using before.