Apparently it's called "The Cosmic Cutie".
This question was also asked on reddit, which links to this FAQ.
According to Douglas, the little green blobby planet thing (nickamed
the "Cosmic Cutie") has been struck the final blow, and will not be
featured on any of the new (American) HHG books. Here's what Adams
had to say:
"I HATE the little green blob and have spent years locked in arguments
with my publishers with me trying to get rid of the obscene little thing.
I've finally secured its demise with the new Ballantine editons of the soft
Non-American readers are perhaps unaware of this book adornment, as it
was only the American publishing houses that determined that without a
consistent motif, all of us Yanks would become hopelessly confused by a
series of books with different names.
It's also mentioned on a Wikipedia talk page. It's mentioned on Topless Robot, which does not provide a source, and The Geek Twins, which in turn links to this FAQ (thanks DavidW).