The role of the media is to be an objective reporter of information to the public. When anyone lies as a matter of course or as strategy it is the responsibility of the media to report the fact that person is telling a lies and to hold them accountable for lies they choose to tell the public. Donald Trump is no different.
Reporters are supposed to inform the people and protect the people from corrupt leadership. To do this, they need to let people know when their leaders are corrupt. In this case, Donald Trump lies constantly, often repeating the same lies. The media needs to acknowledge that he is lying, every time he lies. When he repeats a lie, the media needs to explain that it is a lie.
Yes, the media should call a lie a lie, regardless of who says it. If there is valid proof that Donald Trump has made statements that are untrue and the media has the sources to back it up, they have every right to report this to the American people as a lie.
The media is how people get information. When they simply show what a person says, the people reading or watching will assume that it is true. However, if the media digs for the truth and relays this truth to the viewers, people will know that these are just lies and the person telling them cannot be trusted. This should have started a long time ago with Trump and his lies!