Most are quite aware. Desktop Mode is needed for certain tasks like installing additional software or browsing the web, and it's just KDE Plasma. Also you sometimes need to wrestle with Proton versions and some games with anticheat don't work in Proton, so the average Deck user is pretty aware of what they're running. There's a big culture of tweaking things to run better or use less battery or be more convenient (plugins to make changing game art easier, especially important for non-Steam games you add which have no art set by default).
I could be in a bubble, but it seems like even the less technical Deck owners can at least follow Reddit or YouTube tutorials (of which there are many) to do more advanced things on their Deck. Setting up emulators is one of the more common things people do.
The OS also comes up in comparisons to competing PC handhelds, as the marginally more powerful alternatives all seem to come with Windows and suffer in the UX department. Valve allegedly plans to release their current SteamOS version for others to use, but it hasn't happened yet. There are a couple third party recreations in the meantime that people put on these other devices themselves.
