This term is usually used in a friendly way. Sometimes, the person will not know the name of the individual wearing L.A.M.B., so 'lambie' is just a friendly term to indicate that they also love L.A.M.B. and find it cool that you're wearing an item of it.
Girl with L.A.M.B. bag: Ah, that's so cool...thanks! Btw, I love your Fancy Pants City tee shirt, lambie!