The OL is an issue but Jones is not playing well enough.
The issue isn’t that Mac Jones isn’t good enough (generally). The issue is that Jones is particularly and consistently bad under certain circumstances.
Moving in the pocket, when blitzed, throwing intermediate passes outside the hashes, in critical game situations, when in-game pressure strikes. I know it’s subjective speculation but he really seems to be a sensitive guy and a front runner; when things are going well, he looks good. When adversity strikes he loses his edge and becomes a shell of himself.
Under perfect circumstances, with a clean pocket, when playing from ahead, when the game is comfortable, he’s a very accurate passer. Under any adversity, he falls apart, mentally and mechanically. And that’s ignoring his many physical limitations (which can be overcome but are still present).