Answer:
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.