I tried getting mono installed on Raspbian the other night (Raspberry Pi 3), but it ended up borking my Raspbian installation. I think it was because I wasn’t attempting to install the complete version. However, I’m opting not to continue trying for the time being, as it is fairly time consuming. I have other Pi projects on my plate, ones that are working better and don’t require the same degree of dubious setup.
A big challenge for such an effort would be getting OpenTK compiled for the system after you have mono installed. SDL2 is not a problem, I’ve compiled and installed the latest version of that multiple times. Of course, if mono can just run the standard OpenTK dll’s without re-compiling them, then there would be no problem. But you would likely have to make certain that the associated libraries were accessible on the Pi. (such as SDL2) I would try looking into the OpenTK dependencies.