Whether an average user can use a Linux smartphone depends largely on what the user does with a smartphone and which interface is installed on the smartphone.
Those who need proprietary apps on a smartphone will have their difficulties with Linux. However, if you are looking for a simple phone that comes with a dialer, a few other apps and a web browser, Linux is probably the way to go.
The following list is based on my own experience with the various graphical Interfaces and contains personal opinions and bias which may differ from other users. If I have wronged any project or offended anyone's opinion, it was not my intention to do so. I recommend everyone to make their own impression of the different projects.
Plasma-Mobile: As much as I love the plasma-mobile project, it is relatively unstable. the interface crashes constantly and is generally rather unreliable. Therefore I can't recommend plasma-mobile to "normal" users.
Phosh: Phosh works relatively stable now. The only problem I have with phosh are the gnome apps. They just don't suit my personal taste - no matter how much I try to like them. Other than that, Phosh seems to be a good option for "normies".
Ubuntu Touch: Many of us long-time Linux users like to run away from Ubuntu Touch. This is because the system is read-only and the terminal is more or less useless. Although you can remove the write protection, but that could harbor problems with OTA updates. The big advantage of Ubuntu Touch is that it is relatively easy to install and it works stable and reliable afterwards. The dedicated Appstore only lists applications optimized for Ubuntu Touch and the community is always friendly and helpful. All of this makes Ubuntu Touch of of ( if not ) the best OS for non-technical users in my opinion.
Sailfish: I have had little experience with Sailfish. Therefore I can't say much about it but it looks like Sailfish is also suitable for beginners.
SXMO: Suckless really sucks if you have no idea about tiling window managers and since most average users have no idea about tiling window managers SXMO will probably overwhelm them.
LuneOS: It looks visually very cool, however it seems to be no longer actively developed and since the last release is from 2019, I can unfortunately not recommend LuneOS ATM.
Hildon/Maemo: The concept is interesting, but Hildon does not seem to be fully mature enough to recommend it to regular users.
Glacier/Nemo: Here it is similar to Hildon. The concept is good, but the interface is still undergoing many changes and is not yet reliable enough for me to recommend it to regular users.
Capyloon: The OS looks really nice at first glance, but I have hardly tested it so far and therefore cannot recommend it.
Whether an average user can use a Linux smartphone depends largely on what the user does with a smartphone and which interface is installed on the smartphone.
Those who need proprietary apps on a smartphone will have their difficulties with Linux. However, if you are looking for a simple phone that comes with a dialer, a few other apps and a web browser, Linux is probably the way to go.
The following list is based on my own experience with the various graphical Interfaces and contains personal opinions and bias which may differ from other users. If I have wronged any project or offended anyone's opinion, it was not my intention to do so. I recommend everyone to make their own impression of the different projects.
Plasma-Mobile: As much as I love the plasma-mobile project, it is relatively unstable. the interface crashes constantly and is generally rather unreliable. Therefore I can't recommend plasma-mobile to "normal" users.
Phosh: Phosh works relatively stable now. The only problem I have with phosh are the gnome apps. They just don't suit my personal taste - no matter how much I try to like them. Other than that, Phosh seems to be a good option for "normies".
Ubuntu Touch: Many of us long-time Linux users like to run away from Ubuntu Touch. This is because the system is read-only and the terminal is more or less useless. Although you can remove the write protection, but that could harbor problems with OTA updates. The big advantage of Ubuntu Touch is that it is relatively easy to install and it works stable and reliable afterwards. The dedicated Appstore only lists applications optimized for Ubuntu Touch and the community is always friendly and helpful. All of this makes Ubuntu Touch of of ( if not ) the best OS for non-technical users in my opinion.
Sailfish: I have had little experience with Sailfish. Therefore I can't say much about it but it looks like Sailfish is also suitable for beginners.
SXMO: Suckless really sucks if you have no idea about tiling window managers and since most average users have no idea about tiling window managers SXMO will probably overwhelm them.
LuneOS: It looks visually very cool, however it seems to be no longer actively developed and since the last release is from 2019, I can unfortunately not recommend LuneOS ATM.
Hildon/Maemo: The concept is interesting, but Hildon does not seem to be fully mature enough to recommend it to regular users.
Glacier/Nemo: Here it is similar to Hildon. The concept is good, but the interface is still undergoing many changes and is not yet reliable enough for me to recommend it to regular users.
Capyloon: The OS looks really nice at first glance, but I have hardly tested it so far and therefore cannot recommend it.