Hey Scott, I feel like saying yes and no at every point! So someone's natural temperament does have an effect on their walk etc, but it doesn't have to. A natural pessimist may well be a more doubting Christian, if their pessimism prevents them grasping the certainty and freeness of God's promises, but it's equally possible for natural pessimists (I know some!) to look continually at the dark side of their circumstances yet firmly grasp that the Lord is reigning and all things are working together for his people's eternal good. I suppose that is because faith grasps the bare Word, disregarding sense and feeling. A believer who feels downcast can still believe redemption here and hereafter. You can fully recognise that the fig tree isn't blossoming and every aspect of life is a disaster zone, while simultaneously rejoicing in the Lord (Hab 3).
I guess we must all know at least someone who seemed to be completely transformed by coming into the liberty of the gospel - no longer withdrawn and reclusive, now eager to meet and talk to the Lord's people about the things of the Lord, eg. On the other hand, grace doesn't give people a personality transplant (even people who are dramatically given liberty tend to settle down afterwards). Bouncy extroverts pre-conversion are probably going to be more or less bouncy extroverts post-conversion.
It might be worth factoring in too that grace works on the faculties of the soul (mind, will, etc), more than on personality traits as such. It changes what is sinful, but isn't necessarily sinful to have this or that natural temperament. So grace enlightens the mind in the knowledge of Christ and renews the will (SC Q31), but a person's mind can be savingly enlightened and their will savingly changed irrespective of whether they can be identified as an optimist or an introvert or whatever.
The diversity of personality types in the church needs to be valued because that is where the diversity of gifts and kinds of service can be seen. (If there were no pessimists, how could optimists exercise their gift of encouragement?!) But to the extent that someone's natural temperament actually contradicts grace and undermines their love/service to others, that person should be praying and looking for grace to subdue those tendencies. If all you do is carp and criticise, you can't excuse that hindering, hampering effect you're having on the brethren and the dishonouring impression you're giving of God, by appealing to your personality type. That may and should alter - usually through familiarity with the Word, believing participation in the sacraments, and penitent prayer, as well as fellowshipping with other believers.
What do you think yourself?