Many critics of cryptozoology often contend that it is a pseudoscience. A pseudoscience is a field of study that claims to be scientific, but the practitioners of that field do not use correct scientific methodology when they are conducting their research.
A good example of pseudoscience is creationism. This is because, ultimately, all scientific theories need to be falsifiable. And the claim that evolution is not true, and that the Earth is only 6,000 years old, is simply not falsifiable. For example, some creationists might claim that the Devil planted fossils in the ground in order to test our faith. The problem with this claim is that there is no way of proving whether or not it is true. For example, God could have created the world 10 minutes ago, and left signs of it being far older than it really is, including false memories that he put inside of our brains. However, we quite simply have no way of testing the validity of this claim. Therefore, this claim is not a scientific hypothesis; It is, rather, a religious belief.
However, I am not, by any means, saying that science is somehow against, or at odds with, religion. Religion and science are 2 separate things, and there is nothing wrong with having religious beliefs. However, trying to scientifically prove that creationism is true is a pseudoscientific practice. Once again, this is because faith and belief are the providence of religion, while falsifiable theories obtained from experiments are the domain of science.
Another example of pseudoscience is ghost-hunting, and paranormal investigations. This is because science normally does not deal with the metaphysical, or the supernatural, since theories involving paranormal topics are usually not falsifiable.
However, there is nothing that is inherently pseudoscientific about cryptozoology. Cryptozoology is simply the search for new species. In fact, even a zoologist looking for a new species of insect can be said to be studying cryptozoology.
Therefore, in conclusion, if cryptozoology is practiced in a purely scientific manner, then it is definitely a valid science, and not a pseudoscience.