I also think Jools is a pretty limited vocalist, and he'd probably be the first to agree, which I suppose is why he gets a lot of 'proper' singers on his albums; but I think his piano playing is good, or good enough for the context (although Dr. John could show him a thing or two) which is as part of the band rather than out-front all the time, and he is at least willing to have a bash at anything on Later. I can't really comment on any of the albums as I don't know them, but to my ears a bit of sloppiness can be a good thing.
All in all, though, a good bloke.
Cheers, Bob