It is very difficult to perform consistently in the Premier League without good fitness. The Premier League is very competitive. And if you want to play well here, you must have good fitness. Dybala also played the full 90 minutes in very few matches in the Argentina team. He played just 25 matches in Serie A last season with Roma. And so I personally think Dybala should not go to any Premier League team. This will further worsen his fitness. He will probably miss most of the season due to injury. Dybala should play in Serie A or Bundesliga.
Fitness is one thing but Dybala needs a coach who trusts him and I think Mourinho is the right guy for him for now. Yes he can try to change that, but he is one of those players who are full of potential and end up on the bench regardless. He needs playtime and even though he did quite well last season coming from the bench, I don't know if that is what he wants. As far as I know Mourinho did it because Dybala suffered from several injuries and he didn't want to risk losing him entirely for the rest of the season and those particularly important games in the Europe League, where he showed what he is able to do. It is complicated for him, but unless he can earn a lot more in a different club, he should probably stay with Mourinho in Roma for the time being.