Yes, I think the ability to imagine other people in our own minds is what allows us to understand them in real life. We often seem lacking in concern or empathy toward our fellow man because we don't feel like we understand them. It is only when we are able to imagine the other person in our minds that we see what they are like and begin to understand them.
From family to friend and even perfect strangers, the power of our minds to imagine, assume, judge and label others plays a major role in our social interactions. One could point to the motivations of many stalkers whom prey on others purely out of some fantasy vision as proof. Interracial or intercultural relations are often effected by false perspectives or irrational fears which are feed by the worst elements of our imaginations.
I think the question is based on a false premise. There's no difference between "real life" and the way we imagine others in our minds. The impressions of others we have in our minds, and what we imagine they're like, is as real as real life gets. How else could we understand others, except by what we imagine to be true of them in our minds?
Yes, our understanding of others in real life is impacted by our ability to imagine others in our minds, because it is important that we are able to put ourselves in someone else's position. Empathy and sympathy is key to appreciating what someone else might be thinking or feeling in any situation.
There is no impact on our understanding of people in real life, because all we ever deal with is the image we have of people in our imaginations/minds. Unfortunately, our senses and social 'interactions' only happen in our minds, and only stimulate our mental images, rather than bypass them or even inform them.