Absolutely not! Poor performance on the MMI or CASPer could jeopardize an otherwise strong application. Preparation is key. Familiarity with the format of the test will help you improve your scores by helping you hone your skills in advance, allow you to remain calm in the face of the high-pressure testing environment, and be acquainted with many different types of scenarios so that you are not caught off guard on testing day. Ultimately, while the format may feel very different than the SAT or the MCAT, the MMI and CASPer are both standardized tests, and extensive practice with the format can always improve your performance.