I don't think Bagley will ever be an all NBA caliber defender but you could see him trying at times and he was making improvements, the team's defense was epicly bad and as one of the last lines of defense he is going to look worse than get guy that blew by his man and Bagley had to pull double duty on. I also think that coaching did not do any of the Kings' players any favors this season on the defensive end.
This is a guy who was catered to his entire career and is only now being asked to put in a full effort and he is a young man playing one of the more physical positions, which is why we've seen players far worse than him over their first 3-4 seasons turn into "desirables" when they reach 26 or so years of age. The only question is can his body hold up. That said, I'm also pretty sure that the elite players play through broken fingers like he has had. We can't treat him like grandma's fine china.