The word "middle-aged girl" is a relatively new word in Chinese. It is usually used to describe some women who have entered middle age but still maintain their girlish mentality or appearance.

On the one hand, the word "middle-aged girl" can be regarded as a compliment to such women. It emphasizes that these women can remain young, lively and energetic as they grow older. This kind of mentality and state is very important for many women, which can help them better cope with the challenges in life and maintain a positive attitude. From this perspective, "middle-aged girl" is a good word.

On the other hand, however, the word "middle-aged girl" may also arouse some people's anxiety or disgust. Some people may find this word derogatory, such as "act young" and "feeling too good about yourself". In this case, "middle-aged girl" has become a word with negative meaning.

Therefore, whether "middle-aged girl" is a good word or a bad word depends on the user's intention and the listener's feelings. If the use of this word is positive and encouraging, then it is a good word; If it is derogatory or offensive, then it is a bad word. As a new vocabulary, its meaning and usage need to be further explored and determined.

Generally speaking, "middle-aged girl" is a relatively new word, and its meaning and usage will vary with different regions, cultures and personal understanding. As a neutral word, its quality depends on the user's intention and the listener's feeling.