tl;dr: go to protondb.com, search for Diablo 4, and then filter the list of results by "Distro" for Ubuntu (and Mint), and try the options that people say worked for them
ProtonDB is a site that aggregates reports from users of Proton, the compatibility layer to let Windows games run on Linux. For lots of games, users will submit reports of their system info (distro, CPU, GPU, driver version, Proton version) and any configuration they needed to do to get that game working on their setup. The usual solutions are switching the Proton version you use and/or commands you add to the custom commands in the configuration for that particular game in the Steam menu.
I just looked at Diablo 4, and specifically filtered for Ubuntu users; I see 22 reports, with the most recent ones being people saying it worked for them. There are also successful reports from people on Linux Mint, which is based on Ubuntu.