It's good to be open and honest as a person, which means not hiding anything about your own personality. Self expression is crucial and important because it differentiates yourself from the rest of the world. It means letting your unique identity shine and showing the world who you are. There are limits in what someone should share, but being known as the mysterious one will lose its charm over time because such a person wouldn't have any character to offer, which is the unknown that others wanted to solve behind that air of mystery in the first place.
This is something that is up to the individual person to decide. I myself am very to myself about things that are my business and don't need to be known however i am an outgoing person when with other people that i am used to being around. It is all your choice whether you decide to be an open book or not.