That is not how public domain works…
Uploading to TikTok does not put the content in the public domain, from TikTok ToS, Section 7.B:
You or the owner of your User Content still own the copyright in User Content sent to us, but by submitting User Content via the Services, you hereby grant us an unconditional irrevocable, non-exclusive, royalty-free, fully transferable, perpetual worldwide licence to use, modify, adapt, reproduce, make derivative works of, publish and/or transmit, and/or distribute and to authorise other users of the Services and other third-parties to view, access, use, download, modify, adapt, reproduce, make derivative works of, publish and/or transmit your User Content in any format and on any platform, either now known or hereinafter invented.
Though the last part does allow derivative work on any platform, I would assume doing porn with it would run afoul of other laws.
Tbh though, that’s quite a crazy license that TikTok grants to anyone for any content uploaded to it 
Actually the EU version of the ToS is a bit more restrained on the license granted:
You also grant to each user of the Platform a non-exclusive, royalty-free, worldwide licence to access and use your content, including to reproduce (e.g. to copy, share or download), adapt or make derivative works (e.g. to include your content in their content) perform and communicate that content to the public (e.g. to display it) using the features and functions of the Platform for entertainment purposes, subject to your Platform settings.
So in the EU you can only publish modified TikTok content on TikTok it seems.