If a girl tells you that she is single and ready to mingle, does mingle here mean that she is ready to “hook up”? ¹
1. That is, to have casual sexual intercourse.
In general, to "mingle" means to mix or combine. You might say, for example, "The blue and red marbles were mingled together", meaning they were mixed.
It is common to talk of people mingling, meaning that they are spending time with others. E.g., "I'm going to go to Sally's party and mingle with other people" or "He mingled with artists and writers."
"Single and ready to mingle" is a stock phrase meaning that a person is no longer in a committed relationship to one other person and so is now looking for another partner. I thought it was pretty obsolete: I haven't heard it in many years until now.
It does not imply that the person is interested in a one-night-stand. The person MIGHT have that in mind, but the phrase itself simply indicates a willingness to date.
It means that the girl is ready to start looking for potential partners again. She is ready to get back into the dating scene and mingle with people she is attracted to.
One could imply that her last break up left her reluctant to get back into the dating scene, and being "ready to mingle" means she is over her mourning period.
It does not necessarily mean she is ready to hook up (i.e. have sexual intercourse).
This interpretation works for people of any sex, too, not just girls.