I think if a girl always says something to belittle herself in front of men, then it is possible that she wants men to praise herself indirectly. Women like to hear good things, especially men's good comments on themselves. If a boy can praise a girl, she will feel that she is really as good as a man and she will be very happy. This is a common fault of women, no, it can't be said that it is a common fault, only that it has the same characteristics. The sweet words spoken by boys may sound particularly false to others, but if they are told to themselves, I believe many girls will forget the falsehood of these words and believe them to satisfy their vanity. Therefore, if girls belittle themselves in front of boys, they will generally say that they are not very beautiful in a joking tone. Generally speaking, men will sympathize with this jade. Even though he may think this girl is not beautiful, he will politely say, no, you are actually beautiful. But if you are too serious, it will be self-defeating. Also, there is a situation where girls don't completely want to be praised by boys. She will also use this to test boys' views and ideas about her and see if boys mean anything to her. Then, maybe the girl will say something unimportant to belittle herself in front of the boy she secretly loves. If boys are interested in girls, they always refute her and belittle themselves. Girls can also take the opportunity to ask further questions, alas, you think I am so good. If girls really want to be with boys, but boys are too shy, you can try this method. But be careful not to overdo it, it will arouse the dislike of boys and think that you are Bai Lianhua, which is too fake.
However, if a girl demeans herself in front of another girl, this situation is rare, because girls are psychologically sensitive. Many girls don't want others to know their shortcomings, or that their lives are not good, because girls are more or less vain. Even if life is really worse than others, they don't want others to know. If they have shortcomings, it is the same, and they don't want others to know. Because if others know, they may talk about themselves behind their backs. Even if this girl is really tight-lipped, you will think that this girl must be laughing at her. So many girls don't want to belittle themselves in front of girls because of this idea. But it's different if it's your girlfriend. Girlfriends are very important to girls. Women will rely more on their girlfriends and trust them more. If a girl demeans herself in front of her best friend, it's normal, maybe it's a mode of getting along with them.
The above views are my own thoughts. Maybe not like you, but I hope I can help you.