I think this was a great learning experience for him, personally. I know the phrase gets tossed around often, but all things considered, it wasn't the worst showing.
He played to his strengths, pressed the action (when fresh) and landed some big shots, but Stipe ate them. What he couldn't handle was Stipe's wrestling/grinding, which at that weight, is doubly gruesome and known for draining the sh*t out of guys. I'd even argue that controlling a guy as big and strong as Ngannou, drained Stipe, too. Couple this to the fact that he couldn't put him away with shots, as Ngannou kept eating those, as well.
Meanwhile, Ngannou also managed to work from some difficult positions (earlier on, when he could), while staying in the fight despite a less-than-stellar corner. He definitely needs more experience with his grappling, but if he's going 5 rounds with Stipe, given a somewhat limited skillset, then I see potential.
It depends how bad he wants it and how much he's willing to invest in himself. He needs to switch to one of the top gyms and put some work in, now that he's higher profile.