Agreed. Casual fans already have a hard time telling the difference between Kermit and Robin. Adding a character specifically designed to look like Kermit would just add to the confusion.
I also don't think they'll be friends with Constantine in the end. Don't get me wrong, Constantine is a great character, but, well, if he ended up good in the end, the ending would be a little like this:
Kermit: "Hey guys! You know that frog that stole my identity, almost destroyed the Muppets, and nearly killed us all? It's okay! We're friends now! He can be part of the show! His knack can be juggling."