Imagine your friend that does not know anything about linux, don't you think this would make them not install the firefox flatpak and potentially think that linux is unsafe?
I ask this because I believe we must be careful and make small changes to welcome new users in the future, we have to make them as much comfortable as possible when experimenting with a new O.S
I believe this warning could have a less alarming design, saying something like "This app can use elevated permissions. What does this mean?" with the "What does this mean?" text as a clickable URL that shows the user that this may cause security risks. I mean, is kind of a contradiction to have "verified" on the app and a red warning saying "Potentially unsafe", the user will think "well, should I trust this or not??"
Which is why I said "linux as a whole". Many distros will try to undo the nerdery and neckbeardism that is built into the parent distros but as a whole, linux is going to always be less welcoming to a new user than someone that's used to useless warnings and repeated password entries for elevated privileges. Being safer and being new-user-friendly rarely go hand in hand.
Not all user friendly distros have a parent distro. Checkout Solus.
There are sometimes things upstream causing problems. The Linux kernel itself isn't one of them though as Linus is pretty adamant that Linux distributions should be easy to setup and use. KDE is also designed to be pretty friendly while being customizable still. The main issues seem to come from apps and distributions.