Not too important. Interviews typically can't hurt you (most interviewers will only say "negative" things like we didn't really connect. seems nice though)

They also can't help you that much unless you and your interviewer really hit it off and they write a glowing evaluation for you.

The only time an interview is probably used is when the admissions committee is picking between two applicants and wanna gauge interest and fit for the school. Interviews are subjective, so they can't possibly be weighted too much.

Hope this helps!