The similarity between Mendix and Polarion is that they both have a ALM solution to manage your backlog. The similarities end there. In a nutshell, Mendix is a low-code software platform that enables you to build, test, deploy and iterate on apps. Polarion on the other hand provides tools for requirements management, quality assurance, and application lifecycle management (ALM) of software-intensive products.
Please let me know if your question is answered.
I'm not that familiar with polarion myself. But i did see this post recently on linkedIn from Tom.
Maybe it's worth reaching out to Tom to understand how you can leverage both platforms.