Can be used in a complimentary, insulting, or generic way.
Generally it is used to convey a level of awesomeness.
A starfish on a turkey is not a common sight and so this phrase should not be used lightly. It is used in moments of extreme surprise and overwhelming awesomeness.
"I was not expecting that book to be so good, it was like a starfish on a turkey" (Only proper if you expected the book to be horrible and ended up being amazing beyond words.)
"STARFISH ON A TURKEY! You really startled me!"