Dont know if Retroarch Mupen 64 core is messed up because I got the screen fit in the bezel border without black lines by using ctr shade and linaer shade. it looked so good but it makes games slow drop maybe half of fps and the graphic is off.
running game boy and snesThe icon is provided by the menu driver (in your case probably ozone).
I'm not sure whether it is mapped by the default core (if you set one up) or the playlist name but to be sure, playlists should follow Libretro's naming scheme.
Example:
Nintendo - Game Boy.lpl
Nintendo - Super Nintendo Entertainment System.lpl
Because I have no use out of retroarch on my switch unless said specific game would work perfectly in MP.Why not try yourself?