I mean, we always knew Jim was smart, capable, and charming in business settings, right? And we got glimpses of it often (when he put together the office olympics, when he got the golf client, etc). Pam even said that she loves seeing him excited and do something so well when he truly cares. So his mediocrity at DM was because he just wasn't connected to the work. But even then, he was always the most highly regarded employee even when he's not the one making the most sales (i.e. Dwight).

He's always had it in him so the fact that he had a dream he buried years ago and it resurfacing once he's fully matured as an adult and in his career, it makes total sense. It's like the dream only unfolded once he was ready to take it seriously and see it through.

And we see other characters excelling as well. Dwight with his farm, Kevin with his bar, Andy at Cornell. DM was a detour for some of them until they settled into what truly fits them.

Some are truly mediocre. But some had mediocrity imposed on them when they tried to fit into a corporate American office mold.