I can't help with an explanation of the expression "you'm a pie can", but if used within my vicinity I would have understood it implicitly. I would group it with calling someone a "pie face" or "pie crust".
I assume the origin has something in common with such good Northern English expressions as "he's as daft as a brush" or "as thick as two short planks" - likening stupid behaviour to that of an inanimate nonthinking object.
Any comments?
Quack!!!