I watched the whole debate, and it was really bad. I will say the first 15-20 minutes were the worst, where Biden had a "deer in the headlights" look on his face and was pretty much incoherent, while Trump seemed energetic and on point.
However, as it went on Trump became more and more unhinged. Biden was never particularly good, but it was a lot of disjointed thoughts and stumbling over talking points. I think his debate team pumped him full of stats and he struggled to fit them into a coherent argument. We know the man has a stutter, and it was a struggle, but at the same time he mostly knew what he was talking about and trying to say. So ultimately it was a really bad performance, but some people are definitely overstating how "gone" he actually is.
Should also add that he had a cold, so in addition to everything else he sounded funny as well. Which as we all know, sucks when the stakes are high for a live performance, but at the same time happens to every single fucking person from time to time.