Is a pessimistic person bad?

It is what it is. Good or bad is irrelevant in the absolute sense. But if the pessimism in a person is strong enough that it causes them pain and even stress induced illness, then it's a bad influence on themselves. Also people do not like to hang around people with an always pessimistic view. It is a downer. That could cause a person isolation and again, not good for them.
That is true of any extreme ideals and beliefs.