Both things can be true, the prospect pool was both overrated and poorly developed.
Much of the hype was from having a #5 overall pick, the problem is the #5 pick ended up being more like a #45 pick, and the red flags were there well before he was ever in the Kings system. His freshman year was a huge problem that many here just could never accept, #5 OA picks with 1C ceilings are not supposed to just completely collapse offensively like that at the NCAA level, and the same thing happened the next year in the AHL (long stretches of no offensive production at all). The development decisions sucked, but those development decisions probably ended up hurting his chances of hitting what we found out in the year after drafting was more like a 3C ceiling rather than the 1C one you’d expect from a pick that high. And the fact that he only has 32 NHL games and now projects as a bottom six winger is probably the result of poor development, but poor development isn't why he didn't achieve what is expected of a #5 OA pick.
You also have two other first round picks who, while developed poorly were probably never going to ever amount to what you'd expect from picks that high (Kupari and Bjornfot). Development mistakes can hurt a player, but they aren’t making a top pairing or even a 2nd pairing defender into a waiver wire player or a potential middle six center into Nick Shore. It's more likely Bjornfot's ceiling was a #5 or #6 d-man and Kupari's was as a 4C. That was not the expectation when they were drafted or at their peak value as prospects.
A lot of the excitement was also with JAD, who made the Kings out of camp as a junior eligible player. I think even with ideal development and deployment it was more than likely that JAD was just a guy that peaked early and was going to be at best a bottom line guy, instead of the middle 6 Swiss Army knife type they many of us hoped.
Look, I love to pour on Blake as much as any, and the fact that these guys are all fringe NHL'ers is mostly because of him, but it's not because of him they aren't a scoring line center, great middle six forwards and a 2nd pairing defender. That wasn't going to happen with any oprganization.