Yes, Mendix is most definitely an option for professionals to be a freelancer. But only after you have got enough years of experience as a Mendix developer. Most interesting Mendix projects get done by Mendix partners having their steady crew. Currently, all crew-members are occupied and thus freelancers get hired. That can change, and freelancers will be out of a job.

If you want to become a freelance Mendix consultant, the path of least resistence is first becoming a Mendix consultant in your current job.