He's far from his prime in my opinion, and perhaps he may never reach it. Dembele is by far the most talented of all the young players in world football right now in my opinion. He has such a natural gift for dribbling, can pass and score with both feet, incredible crossing ability already. But whereas with Mbappe I'm convinced he will deliver on his promise, there's still a lot of question marks over Dembele. His mentality is the only thing that can hold him back besides injuries. This is a guy who threatened to retire and live in Senegal a year and a half ago. He's found a good place in Dortmund to develop, and even if he is unhappy with the firing of Tuschel, he seems set on staying in Dortmund (which he confirmed in a French interview right after the England friendly).
The point is, I think he needs a stable environment with a coach that has patience and can develop young talents because he can be frustrating just as much as he can be brilliant in a game and I'm not sure Mourinho is the best coach to handle him. He's someone who right now can create a moment of magic in every game and be decisive with an assist or a goal, but he's also prone to a lot of stupid passes and questionable decisions. If he's moving anywhere in the summer, it's not to Barcelona because he wants to be a starter, which he would be in Dortmund.