YES! Unless there is a special circumstance, like graduated college 10 years ago, taking 6 months for primarily MCAT would be extremely frowned upon and considered the antithesis of what doctors should be able to do (be relatively selfless, multi-task). When it is time to study for Step exams, which are much more content-driven and much more content, you will get much less dedicated studying time, if any, at most medical schools. Plus, the new mcat is much less content and much more an aptitude-type test.
Others are correct in the ADCOMS might not be able to figure out that you spent 6 months studying for MCAT. But you would certainly never want to lie or mislead any who asked. There are certainly plenty of things that you could do during the 6 months, including significant volunteering, shadowing, working in a job to earn some money, etc that would teach you important life lessons, bring you joy, etc