The typeface makes the articles look like typewritten manuscripts rather than published articles,
Courier is being used only for headings, not for the body text. I find that it looks better than Noto Sans, and it works better with horizontal lines. When I tried Noto Sans, its descenders touched the horizontal line beneath the H2 heading, and the lines in the H5 heading were not the same distance from the text. I also dislike how barren and uninteresting sans serif fonts look when used for headings. I find that using a non-proportional font for the headings makes them stand out a little bit better and distinguishes them from the body text more.
and having underlining in the headers looks weird and unproffessional.
This is a hobbyist site, weird is a personal opinion, and there are only so many different ways of distinguishing six levels of headings by style.
Courier is being used only for headings, not for the body text. I find that it looks better than Noto Sans, and it works better with horizontal lines. When I tried Noto Sans, its descenders touched the horizontal line beneath the H2 heading, and the lines in the H5 heading were not the same distance from the text. I also dislike how barren and uninteresting sans serif fonts look when used for headings. I find that using a non-proportional font for the headings makes them stand out a little bit better and distinguishes them from the body text more.
This is a hobbyist site, weird is a personal opinion, and there are only so many different ways of distinguishing six levels of headings by style.