Most of the use of a smartphone is in public, so it is less likely used for smut than someone using their computer in private.
i dont know who your are talking about, but I know that there are plenty of kids who have smartphones and use them in private, all the time, for “shmutz”. maybe if one is a single living by them selves then a computer is as dangerous as a smartphone, although you would still have access in private places that are not at home. but for many other people, especially teens, a portable, always there, private smartphone, is far worse than a in place, perhaps supervised, public computer.
whatever the case, a person should definitely have a filter and reporting service on their device. both of them.