@ehh122302 Fortunately for you, there will be very few applicants with a better essay score. Your essay score is not mediocre, it is, in fact, a very good score. It is in the higher range of the mid 50% for Berkeley and UCLA, which are the most competitive colleges which require the SAT essay scores. For most colleges which require the SAT essay, your scores on in the top 25%.
While the 5 is a bit low (but well above average for that subscore), having an 8 on any subscore is extremely rare (fewer than 1%). So, paired with your SAT scores which are in the top 1%, I would say that your SAT essay score is something about which you should not worry.