Defensively he's clearly capable, just very inconsistent and prone to lapses in concentration.
An example of inconsistency: I've seen him make perfectly timed, crunching sliding tackles on people many times. But I've also seen him attempt that playground-style back foot tackle as someone races past him, something that no serious defender would ever do
I don't think he makes an unusually high number of actual defensive mistakes for a modern, attacking fullback. By defensive mistakes, I mean things like positioning, covering, marking, tackling etc. His mistakes are nearly always due to poor concentration or bad decision making
Stones used to have that problem. People would say it was "another defensive mistake" by John Stones but it was very rarely a purely defensive mistake. Usually he'd play an errant pass that got us into trouble or misjudge the flight of a ball having positioned himself correctly to deal with the danger. Regular mistakes in defence, rather than defensive mistakes
Eric Garcia, for example, made actual defensive mistakes all the time. He was never where he was supposed to be, he let runners go, he got caught the wrong side etc.