Lamb is becoming more and more accessible around here. Used to be, if you wanted lamb, you needed to know a farmer. Sometimes he'd raise it for you and you had to butcher it yourself. I know people who do that with goats.
I use a combination of ground beef, ground pork, ground veal, and ground lamb for meatloaf. And I use a combination of ground lamb and ground pork for Scotch eggs. I've never had a lambburger but I expect it would be just fine.
eta: But, yeah, I think I'd find something besides ketchup to put on it.
I believe putting ketchup on ground lamb is a hangin' offense in some parts.
Now, mustard, on the other hand...