Well, writing aside, the artwork from Robin Hood, I won't say was bad (certainly not as messy as the Muppets take Manhattan comic adaption... the one with Purple Rowlf on the cover), just... it didn't know what it wanted to be. It's like it was trying to be Muppet/manga/middle ages/Roger Langridge Muppet/action style all at once, and didn't manage to be any of them, or all of them too much at once. I really think if the artist were able to practice his Muppets and had a lot more experience, he would have nailed it. Plus, the Medieval style came off as muddy and boring looking. I know what the guy was trying to do, but I don't think he quite accomplished what he wanted. Plus, only about half the characters turned out good. I think his Johnny and Sal was very good, and that one time he drew Clifford in a close up was alright... but it seemed that he wanted the same style of Gonzo and Kermit that was in Langridge's comic. And that's where I think things fell apart a little.
As for the writing, I think the problem is explained well in the 4th issue... there's just TOOO much legend of Robin Hood to fit in 4 issues, and the writing tried to cram WAAAY too much into them. Compare it with, say... Peter Pan... things in that comic were either shortened or omitted entirely (no mermaids, the Indian segment was to be longer... and Swedish, and no Crocodile)... but it worked BECAUSE it didn't need any of them to tell the story. The comic was just as good without them, probably better... I didn't miss them because there was so much else to enjoy.
King Arthur really seems to take it the same way. The legend is clearly boiled down to parody, and that's what works the best. After all, the legends (there are more than one) of King Arthur all basically contradict each other (they seem to have nicely blended the Lady of the Lake AND the Sword in the Stone legends), and half the characters were invented for epic poetry and stories... Lance-a-lot wasn't even an original knight. He was added in by a French writer. And as such, a story that isn't even consistent to any one version, adaptation makes the thing work all the better.