Hello!
Ok, I really regret naming that property "quality" but that value refers to font point size. If you set it too high, the font will naturally be way too sharp and use Unity aliasing to resolve itself when displayed that small, so it can look like that.
So i suggest lowering the "quality" value and seeing how that works for you. If you're using STM on Unity UI, there's an option to automatically set this value where it tries to get the best value for you, but generally... if your "size" is 40 and this is on a canvas with no scaling, set the "quality" to 40 to have a font size that matches the canvas.
For a true SDF, you'll need to follow the steps in the documentation to generate an SDF font, but usually the steps I just listed are good enought o have great looking text.