No, pessimism should not be added to the list of religions, because there is not a deity in pessimism. At best, pessimism is a feeling, or a personality trait. It is not a doctrine. Nor do pessimists tend to get together for fellowship. Pessimists tend to disagree on everything, except complaining about things.
No, pessimism, to me at least, is a way of thinking. It's not a religion. They do not believe in a higher power, as far as I know, that is. They simply are pessimists and even some Christians are pessimists as are some Muslims. They should get no type of tax break for being pessimists.
I do not believe pessimism belongs on the list of religions. Pessimism is simply a state of mind. People can be pessimist and non-religious, in fact this is quite common since people who are pessimistic tend to be nihilist who see no intrinsic worth in life anyway. Pessimism will never fit the definition of a religion.
Pessimism is a state of mind or perception of reality. Religion is a belief or formalized doctrine in which a person or group or individuals adheres to. Pessimism is merely the way an individual chooses to see the world in which they live and does not invoke any real dogma or ideas outside of that evil or hardship will always happen more that good or comfort.