In my admittedly limited experience, people who start off relatively normal are unlikely to get to the point where being too mellow would be a problem by way of meditating. Without a concerted effort to make it do so, meditation doesn't seem inclined to change peoples' default mental states; most likely you'll wind up with the meditation-related states as voluntarily-achievable extras, if anything. (Some people don't even manage that.)

If you do tend to naturally find yourself in the kinds of states that most people try to achieve by meditating, or you tend to have particularly volatile default states, it might be worth worrying about, but in at least the first of those cases you're probably not going to get too much from meditating in the first place.