No, how would that be rude? It obviously depends on the context. If that person is with their friends, family, etc., and everyone is socializing except them, and honestly, yes, it's like someone spending all day on their cell phone instead of socializing. Technically, the question would be, if they wanted to read, why did they go there?
I can tell that kind of person is indifferent or anti - social. It's not good to see him or her reading while others are engaging with something that connects them. That's rude.
I don't think reading can be considered rude, however, if you are among people and you start reading instead of interacting with people it might be considered rude behavior.
That kind of person reading while others are busy talking and laughing that person might be introvert. Introverts don't like noise. They like to be alone however, they can control their attitude.