Being a doormat is not about being timid in everyday life, it's more along the lines of a submissive who gives up thier submission all too freely. They ignore thier limits, they only exist to please thier Dominant, never taking responsibility for thier own actions, choices or happiness.
Basically a doormat submissive is a robot submissive.

A submissive should know within thier relationship with thier Dominant when to be submissive and when to be a companion.

I am an extremely shy person, prone to avoiding most social interactions with others. I don't like to stand out in a crowd, but prefer to blend in and stay quiet. I still have a job, and other interests which I enjoy.
That isn't saying I can't have a relationship with another person and be submissive to them.
I make the choices to be with another Dom, I set my limits and do what makes me happiest. I don't do what another tells me to do, because I am submissive.