This is an interesting question because it suggests that there might be a link between the length of a domestic cat’s hair and their personality. To answer the question I think you have to do look at random bred cats and purebred cats separately. The question is curtailed because it is meant to be comparing long-haired cats with what? Short-haired cats I guess.
Random bred cats
There is no connection genetically speaking between the length of a cat’s hair and their personality, to the best of my knowledge. Is there a connection with the color of their hair and character? However, it is conceivable that over many years there has been what I would describe as informal selective breeding. It’s possible that people who decide to adopt a long-haired random bred cat from say a rescue centre select those cats that have mild-mannered personalities. It may be the case that people who like long-haired cats also like mild-mannered cats. In fact this is very feasible.
If people adopt long-haired random bred cats who are mild-mannered you create an informal selective breeding process. The more fiesty long-haired cats at rescue centres or are unadopted and therefore they are possibly euthanised – culled from the pool of breeding cats. Therefore over perhaps a hundred years you may end up with random bred long-haired cats who are slightly more mild-mannered than short-haired cats. Of course I am speculating and there is no scientific evidence to support this as far as I know. It is certainly possible.
It is far more likely that long-haired cats are more mild-mannered than short-haired cats within the world of purebred domestic cats. This is because all purebred cats are selectively bred and breeders will breed for appearance and character. The two go together. For example, the Persian cat is very long-haired and breeders like to breed these cats with placid, docile characters. It’s part of their overall characteristics. This is one good example of how personality and length of hair go together as a package.
The Ragdoll cat has long hair as well and they are known to be very docile and mild-mannered. Once again this is down to selective breeding. The Maine Coon has long hair. Maine Coons are described as relaxed and easy-going. They are people-orientated. Therefore relative to say a Siamese cat they would be described as mild-mannered.
In other words the question in the title is really asking whether long-haired cats are more mild-mannered than shorthaired cats. This means that we are talking about a relative difference. And sometimes they are.
Dr Yuki Hattori suggests that slender, short-haired breeds such as the Abyssinian and the Russian blue are often lively. Certainly, the Abyssinian is a quite active and lively cat as is the Bengal. He also suggests that long-haired cats such as the Persian tend to have more mild-mannered natures. He’s a respected cat behaviourist and for the reasons I discussed above it is possibly true that there may be a tendency for long-haired cats to be more mild-mannered than the shorthairs.
However, there must be a big caveat on this discussion which is that each cat has an individual character. It is difficult and perhaps unwise to bundle a group of cats together by the length of their hair and say that that group is more mild-mannered than another group. It depends upon the individual cat at the end of the day. And there are many factors which dictate a cat’s character, a major factor of course being how they were raised in the environment in which they live. I am sure that you’ll find Ragdoll cats that are not docile and Persians who are difficult.