Ugly master's thesis but one great proof I wrote an ugly master's thesis in mathematics. There were errors and I think it is not worth publishing. However, I managed to prove one theorem in a simple way that I have never seen before. I have seen another proof of the theorem and, in my opinion, it is more complicated than my proof.


What would be a suitable way to get other mathematicians to know the proof? 
Is it fine if I post a question to Mathematics Stack Exchange and answer my own question?
What would be a suitable way to get other mathematicians to know the proof? 


Publish it!  Write the proof by itself in a short, self-contained paper, and submit it to a journal.

There are lots of papers in the literature giving "A new proof of ...".  You may not be able to publish it in a very top journal, but if it's really new and significantly simpler than existing proofs, some reasonable journal should accept it.

Additionally, you can post it on https://arxiv.org, so that it's permanently available to the community, whether or not it eventually gets published.