If you fail the mock lesson you will not get the job with what ever company you are applying to so passing the mock lesson is essential for your success. But there is also another really important reason why you need not just to pass the mock lesson but why you need to pass it well, and that is the salary.
How well you pass the mock lesson, what score the Interviewer has given you after they have seen your mock lesson will determine your lesson rate meaning how much you will be paid by the ESL company. This is really important as most ESL companies do not give lesson rate increases once you have been hired, so it is important to nail a good lesson rate from the start.