Paper based on Master Thesis - Disclaimer necessary?
Yes, this is necessary.

Self Plagiarism may seem like a strange thing to be worried about, but there are many reasons to avoid it the way you would any other plagiarism:


Some journals use double blind reviewing, so reviewers can't distinguish between self-plagiarism and plagiarism.
You may have received funding for your thesis, which is probably acknowledged in your thesis, but not necessarily in the paper version.
People reading your paper may want to read the full Master's thesis, since theses generally contain more detail and are less space-constrained than papers.