Playing Epic games on the Steam Deck has always been a bit of work, but it is possible to do. You would need to go through the Epic Launcher in Desktop Mode and add it to Steam, or you could use the Heroic Launcher and add games directly to Steam, but it does still require you to go to Desktop Mode. You can add the launchers directly to Steam to access from Game Mode too, which works, but it will be significantly harder to use due to incorrect scaling. However, there is a new plugin that just released that seems to bridge this gap, bringing Epic games directly into Steam in a more uniform and easier-to-access way. That is the goal of Junk-Store.
Junk-Store, created by ebenbruyns and their team, was just released into the wild with its 1.0 release, and it is pretty amazing seeing the work that was done. By going into the plugin, you can log into Epic and get a list of your games shown to you, similar to how Steam shows you its library. Then, all you have to do is pick and choose the game you want to install, and there you go! It's just that easy and all in an incredible and clean interface.