Stadia now allows you to , provided you're willing to try the functionality out as part of Google's latest experiment. You can enable the feature by launching the Stadia app on your phone, tapping on your avatar, and navigating to the Experiments section of the settings menu. Keep in mind Google is currently rolling out the feature to users, so you may not see the option right away.
When Stadia launched in 2019, the only way to use the service over a cellular connection was to use tethering to trick the app into thinking it was connected to the internet over WiFi. By contrast, Microsoft's competing Project xCloud service has offered cellular streaming since day one.