Tomoko has poor self-esteem and poor social skills.
Here are a few reasons why for you to choose from:
1: Adolescence is a time when a person is pushed and pulled by a lot of social forces. Such as the desire to fit in, the desire to be unique, the desire to rebel. Everyone is competing to climb the social pyramid, and so consequently, some are like Tomoko, and left at the bottom.
2: Tomoko's hobbies are more orientated to an isolationist lifestyle. This has possibly led to her social development being stunted, making it harder for her to speak and socialise with people outside her bubble.
3: Tomoko spends most of her time playing dating simulators and looking at porn, causing her to develop unrealistic expectations about relationships and social interactions. This gives her a harder time adapting to the dynamics of real life interaction, and can cause her to feel disappointed, rejected, or inferior when her expectations are not met.
4: Tomoko moved up to high school with no acquaintances from her previous school, putting her at an immediate disadvantage to the rest of her classmates, who all seemed to have someone they clicked with straight away.
5: Tomoko may have mental health problems, such as social anxiety disorder, autism, or aspergers, that causes her to experience a much greater deal of stress and discomfort when in a social setting.
6: Tomoko is a fictional character, created to be socially awkward on purpose so that people that might fit the above descriptions can relate to her.