A girl sent a text message in a joking tone: If no one wants me in the future, I will follow you~ What does that mean?

Absolutely no joke.

No matter how careless a girl is, she would never make such a joke about something like this.

99.99% probability is that she likes you but is not sure about your attitude, so she gives you a try.

My suggestion: If you like her, you can take the initiative. After all, girls are not likely to take the initiative. It is not easy for her to say such things; if you don’t like her, make it clear. Well, don't give her fantasies and let her get into the realm of fantasy.