Ish. It's less about mean vs nice and more about confidence. You can be nice and confident and the ladies (or gentlemen if you're into guys) will really appreciate that.
As a former 'nice guy' there's nothing nice about pining for someone and then getting bitter / entitled when they choose someone else over us. There's nothing wrong with feeling down after a rejection, but it's definitely unhealthy to blame it on them or to dwell on whether or not we 'deserved' a different response.
Word of advice, have more confidence in yourself. You are awesome, and if you acknowledge that, and if you help create more positive experiences around you as you venture through life you will find that others will recognize you for it.
Rejection isn't really a reflection of your self worth anyway, it's more due to that person's situation and personal preferences. Life gets easier when we stop taking rejection personally.