Not sure why you'd answer to people calling Manny an "average or even poor" DC. As a DC, he was above average and it's hard to argue anything beneath that level.
Currently, despite the fact he's not our DC, but rather our HC/DC, the real debate seems to be around whether he is above average or elite, and how that relates to the execution of his defensive philosophy. And, for that, I think you'll need more than YPP.
I don't question your conclusion based on the narrow scope you selected. But, the methodology tells me there may be a separate discussion/analysis to have. See, part of the problem we've watched unfold is Diaz's philosophy is designed to be good in that YPP metric (btw, I think it's a good indicator, too, but often needs more context).
He coordinates for negative plays to get an offense off schedule. Those negative plays come at a risk, which as a DC he often did a good job of mitigating. The result is a nice YPP metric. However, the philosophy is vulnerable, as we saw in multiple instances or when he (or Baker) is missing a piece. That said, his most impressive year was when he combined a strong YPP with elite 3rd down %. I thought he was on his way to really evolving his defense at that point, and then we saw a disjointed philosophy the last two years. Sure, it was "Baker," but it was Manny's defense and he said himself he was in on everything.
I think the more important question is if his philosophy, when executed as it has been in the past, is solid enough for us to win bigger games consistently. I would be curious to see an analysis of how he performed in "big games" (however way you want to define that) vs how other coordinators/philosophies performed in "big games." Say, Top 25 teams. If there are common Top 25 opponents, all the better.
All of the above is moot if he tinkers with his philosophy and allows Jess Simpson, Shoop, and T-Rob to offer different perspectives to come up with a more controlled version of the negative play madness, and one that has the appropriate coverage behind it. For instance, we played a ton more zone behind the madness these past two years under Baker, resulting in obvious vulnerabilities.