We only ever saw Finn up to when he was 17. He was also an kid for a good part of the show. PB basically manipulated him into being her errand boy.
Mordecai, on the other hand, was a grown man who even then acted like a complete doormat for margaret. So for example, that time where killed rigby (He literally pushed him into the void after screaming "I'LL KILL YOU", i know it was an moment of anger but that's still not a good thing), or the way he acts with Margaret, constantly throwing everyone, himself included, under the bus so he can impress her. The time he smelled her sweater and went on a big-ass trip which ended in a nasty fight with Rigby, that time he kept messing with time because he ate mexican food before they kissed, etc, etc...
Regardless, simp is a retarded term and we should either create something else or go back to beta males or orbiters.