Is Samuel Taylor Coleridge a pessimist?

Coleridge can be seen as a pessimist in some sense, but in comparison with other writers of the same general period and especially somewhat later, this tendency is muted in his work. The first generation of English Romantics, namely, Wordsworth, Coleridge, and Blake (though Blake is so unusual that he cannot be neatly categorized as a Romantic, and I am including him mainly because he is a contemporary of the other two), were young men during the French Revolution. They absorbed the mindset that mankind could improve itself, and they saw the new age as one of liberty and hope. In the closing stanzas of The Rime of the Ancient Mariner,we can sense this positive orientation:

He prayeth best who loveth best
All things both great and small,
For the dear God who loveth us,
He made and loveth all.

However, the story is fatalistic, as an almost inadvertent act brings about a catastrophe to the Mariner and his ship. This, one could say, is the essence of pessimism, and the poet's aim is to make us all "sadder and wiser" as a result. But it is a dual sentiment, both positive and negative, that concludes the poem. The tone, the philosophy embedded in it, cannot be simplistically delineated.
Though "Kubla Khan" is a fragment, a similar blend of opposing attitudes can be seen in it. This is an exalted vision of both hope and despair. We are told that the pleasure palace, a "deep romantic chasm," was "a savage place":

as holy and enchanted
As e'er beneath a waning moon was haunted
By woman wailing for her demon lover!

The entire poem is a narcotic vision of paradoxes of good and bad.
Coleridge, like many poets, cannot be easily classified. The second generation of Romantics—Byron, Shelley, and Keats—leaned further to a pessimistic orientation than either Coleridge or his friend Wordsworth. Coleridge himself shows, as stated, a mixture of these tendencies in his unique way.