I liked him a lot because he helped complete the scene of a successful spooky Fiver moment. He went full-on creepy with that forceful whispering voice of his. At first I thought his mouth was covered with dried blood (or Fiver's vision was making it look that way), but it appears his mouth is just a darker color. The funny thing is, I thought this scene was always supposed to be creepy (although it confused me why Cowslip would let someone say such depressing scary words to his rabbits), so I'm thinking the scarier the better and he's doing a fantastic job, but then I looked at the book again.
In the book, the poem was longer and sounded more like it was about making death acceptable instead of "ooh watch out, death's gonna get you". The earlier paragraphs put the last paragraph in a more reassuring light. Unfortunately all of the adaptations mostly pull from the last paragraph, so that context is lost and I would always hear it as doom and gloom. The movie to its credit had Cowslip speaking normally. But then the TV show had Silverweed sounding like he was in a trance with a disturbing smile, and the miniseries had him sound menacing.
Personally I prefer the scary interpretation and miniseries version, it's more entertaining and my mind was going to go that way anyway with lines like "I give you my breath, my life", but it still seems different from what the book was aiming for.