It varies in importance based on the school. In some schools your interview performance makes or breaks whether you get accepted. In other schools you get a score and that score is factored in to numerous other graded criteria (your academic performance, your extracurriculars, etc.) and then an admissions decision is based on the total. The weight of the interview depends entirely on the school.
That aside, it's probably safe to say most schools will reject an applicant that fails to show basic social competence at an interview.