I think people who think that Buddhism is pessimistic because they base their views on the First Noble Truth that all conditioned things are in a state of suffering. But Buddha also taught the cause and end to suffering, that is the purpose of this religion.
The Buddha image is that of peace, serenity, and compassion, hence in popular culture, the Buddha image is well sought after to bring peace and calmness to the mind, so how can such a pessimistic religion has that kind of effect? Buddhism is also about the conquest of the Self and overcoming/victory over one’s Mara. This is very positive and is the foundation of self-development .
Sometimes, we have to acknowledge the “ugly truth” so that we can face the reality and move on. Hence Buddha taught the fact of suffering so that people can face the reality of samsaric suffering, then choosing the path to overcome the suffering and moving in the direction of ultimate happiness (enlightenment). We always read that Buddha and Bodhisattvas are filled with joy in the midst of samsaric suffering, always ready to help all sentient beings, so I don’t think Buddhism is pessimistic.
There are two Buddhists texts called the Theragatha and Therigatha which are full of the joyful utterances of the Buddha's disciples, both male and female, who found peace and happiness in life through His Teaching. The king of Kosala once told the Buddha that unlike many a disciple of other religious systems who looked haggard, coarse, pale, emaciated and unprepossessing, His disciples were 'joyful and elated, jubilant and exultant, enjoying the spiritual life, serene, peaceful and living with a gazelle's mind, light-hearted.' The king added that he believed that this healthy disposition was due to the fact that 'these Venerable Ones had certainly realized the great and full significance of the Blessed One's Teachings'(Majjhima Nikaya).
When asked why His disciples, who lived a simple and quiet life with only one meal a day, were so radiant, the Buddha replied: 'They do not repent the past, nor do they brood over the future. They live in the present. Therefore they are radiant. By brooding over the future and repenting the past, fools dry up like green reeds cut down [in the sun]" (Samyutta Nikaya).
One thing for sure, Buddhism encourages us to be realistic: we must learn to see things as they truly are.