Do I need a technical writing degree or certificate to become a technical writer?

From my experience, no, you don't need one to get hired, necessarily.  I would say though that it is helpful to get one, for most people.  You can learn a lot about: audience analysis, fonts, software tools, readability, layout and reader useability.  Perhaps some people have natural interest in these topics, or natural talent, but I think for most they are learned skills and knowledge.