Is it cheating if I develop some part of my MSc. thesis in a hackathon in a team?
I believe it would be fine and acceptable as long as in your thesis writing you name who helped you and how they helped you.

I doubt anyone will think less of your thesis for having people help you. As long as it is still clear that you did a lot of work yourself... this team isn't completely doing your project and thesis for you!

You could ask your thesis advisor to be certain.

Many PhD students hire research assistants to help them code software engineering projects.