I think it's all relative to the cat. Some cats think they're fine by themselves until they get another cat in the house; then they realize what they're missing. Others are completely solitary and avoid the new cat.
Like people, some cats just don't like other cats being in their space. Some are social butterflies who love everyone. Again, all depends on the cat.
Is it MEAN to get another cat? I'm going to answer this in two ways:
First, it's not mean to the other cat to get a new cat. There may be jealousy, hissing, fights at first, but most cats can move past that. When you become a multi cat household, you have be able to commit extra time to the original cat to make sure they understand that, yes, at the end of the day, she'll always be your girl.
Secondly, when you make the decision to bring another cat into your home from a shelter or off the street, that's not mean. That's HEROIC. Congratulations; one less cat died today because of those actions.