You may be right. But I don't think so, and I'd actually use Tammy Abrahams as a prime example. He scored shedloads playing against championship defenders whilst with Bristol City. got found out when in the Premier League with Swansea, and is now doing it again against championship defenders with Villa. For me, that doesn't make him a better player, it makes him a decent championship player. I expect DCL will be just as prolific as Abrahams at that level. But that, in my opinion, won't make him a good premier striker.
DCL needs to improve in all area's of his game, which is fair enough as a 21 yo. There are certain aspects he needs to improve more than others. But if he is going to become a premier striker he needs to be doing that learning against the top defenders. All he'll learn in the championship is how to score in the championship. That may actually be his level; he still has a lot to learn to be a regular starter at a premier team that has European qualification as a standard. and there's no guarantee he'll reach that level. But at least he is still improving albeit a bot slower than we would wish. It would help if we had a top striker at the club that he could learn from.
The bottom line is that he is decent back up. It isn't his fault that our main striker isn't good enough.