Cockatiels have very unique and complicated behavior. As you said that your Cockatiel seems friendly towards everyone but is still hissing then it shows that she might feel uncomfortable or is just seeking attention. It could also be sign of unhappiness or boredom. Try to bond with the bird and keep her happy by giving her good food and toys etc to play with. You can also train your bird for different activities. Just ignore the hissing part and keep loving your beautiful bird.

Its could also mean that she is trying to dominate you what i suggest for this behavior is that you keep her below shoulder level. this would consist of taking her off of your shoulder when she tries to use it as a perch and using a calm but assertive tone when telling her not to do something. If that means risking a bite or two then so be it!

So, my sister got a cockatiel a month ago from a different town and she brought it home with her. From day one, the bird has been very friendly. However, even though she is still friendly, she has started to hiss/scream. Now, this can get quite loud, but it is especially troubling when she hisses while she is on someone. Just a few minutes ago, she started hissing at me again while on my shoulder. She hissed for over a half hour, before finally stopping. She is still hissing on and off. What could that mean? She doesn't seem to have any hostility or anything towards me.