Well, to start with, are we sure Slughorn did it badly, and it's not just a case of, even well-done memory faking just looks bad?

However, assuming they can be down well enough to pass... I like it! I can even see it: Snape is a good guy, but he figured Harry is too thick to just believe that Snape could just reform for goodness' sake (heh), so he came up with the Lily thing to give Harry a tangible reason to believe him.