I'll admit Stones has made mistakes mate, but he also makes a lot less when Jags is next to him.
Mori has done well, and I've been impressed, but he was brought in as a third-choice CB, at least for this season. He's proved he's capable of stepping up though, which is a good sign.
For me, it's just interesting how fans/pundits react when a young player goes through a tricky spell of form - which Stones is now doing for probably the first time. There always seems to be two camps (usually with the same people in each of them).
A - Calls for them to be dropped (a kick up the arse etc/needs a rest). This happened a lot with Lukaku last season, and even more with Barkley (this season, even though he's been one of the best midfielders in the league).
B - Leave them in, let them grow and develop and overcome this bad spell - as it will make them a better player overall. If the mistakes continue for a sustained spell (e.g. five, six games) then maybe look to switch it up. (Basically what Rio Ferdinand said on 5Live last night).
IMO, Option B is preferable, but then again everyone thinks differently.