I *personally* (it really is different for everyone) never imagined April to really look like Mimi. In fact, for some reason, I always envisioned her as a redhead myself.
Roger is having a hard time getting over April. He's still thinking about her. She was the last woman he loved- the last woman he ever flirted with. Now, there's this girl in his apartment flirting with him... he's probably reminded of April just a little bit by some seemingly insignificant things that she does. He picks out her smile as being like April's. They might not actually even BE similar, but in his mind he sees it, because he's kind of obsessed with it.
When you're obsessed enough with something, you see it everywhere (I think hardcore RENTheads can attest to this... I know I can associate ANYTHING with RENT).
"I mean, how many of us could honestly say that at one time or another he hasn't set fire to some great public building?"