This is a very interesting question.
My entire job is talking to people, getting to understand them and trying to help them. in my professional life, some people are easy to work with and some are hard work, you have to drag every piece of information out of them.
In my professional life, the most "boring" people are the ones who don't give me anything. They tell me about how other people treat them, they tell me about what their neighbour said, and they take ten minutes to tell me the story of why the doctor changed their blood pressure medication. They don't tell me about themselves. Sometimes they don't have a strong sense of self and sometimes it's my job to help them think about that....but in real life i have the tools to do that, i'm allowed to ask them questions.
In literature, a character who doesn't tell me about themselves doesn't interest me. As a reader I don't have the tools to find the information about them that would, the author either gives it to me or they don't. If they don't then the character is flat.
I enjoy the process of finding out about people. The most interesting thing someone can tell me about themselves is that they think they're boring, but I can do something with that information in real life. If an author tells me a woman is quiet in their story, then has her act quiet and doesn't share anything about her then quiet quickly becomes boring.
I don't believe that there is a single boring character type, but if the author hasn't put enough thought into what makes a character interesting then there is no way they can give me that information and make them interesting to me.
If the author has created a well rounded character, and then makes good choices about which scenes to show me them in, then they can be interesting. If an author shows me a quiet wallflower in a a party scene, then shows her having a routine day at work, then a scene where she has an early night then I don't get enough from that. Throw in a couple of conversations with a friend where she's sharing her feelings and she's much more interesting....the author can do a lot by letting me see all aspects of a person, not just a couple of repeating behaviour patterns.
Craig