I mean it depends. Xelatex and Lualatex, which are actively maintained, use Latin Modern. If you just use pdflatex then yes it's Computer Modern that gets used.
The typefaces are close enough though that it is quite difficult to notice the difference.
Yep I just changed the font size to 18px in my dev tools and it looks better. Still don't think serif fonts are the best for a blog but guess that's just my opinion.