Last I checked, there's no canon with name Mars. Also, tag wiki indicates that it's for the planet in Sol System.
Do we need this tag? How many people browse with planet name?
Do we need it? Yes.
From the FAQ : What are tags, and how should I use them?
A tag is a keyword or label that categorizes your question with other, similar questions.
Are the questions that are tagged with mars being used correctly? Yes. All of the questions with this tag have questions about films/books and stories that have Mars as a major theme. This includes Andy Weir's "The Martian, Total Recall, Robinson Crusoe on Mars, Kim Stanley Robinson's Mars Trilogy, etc.
Tags can also be used to help you identify questions that are interesting or relevant to you.
This tag is likely to be of interest to people with a special interest in stories about Mars.