I already tried it, still blurry. I don't have retina display.messages has been on the mac for years, a descendant of imessage; it's not a 'port from IOS'; and it's not a universal issue.
is this the only app that happens with? perhaps try changing the font size in messages, then back to what you want...
There is completly redesing Messages app in Big Sur. I tought it's port from iOS.