In 2023, you'll be hard pushed to find a GNOME application actually using C# and Mono. The vast majority of GNOME components are written in C, with a large number of them written in Vala and some in Rust and JavaScript.
It was indeed a big yak to shave. Cloning a Microsoft technology probably wasn't a good idea for OSS adoption either.
I'm sure; there are still threads that come up from time to time in GNU circles about whether Mono is actually FOSS (TLDR: it is), in which those of a more paranoid personality allege an EEE attempt by Microsoft. If this were the case, Microsoft clearly did not succeed even without Java as a legitimate competitor! Interestingly, Mono has had most success from its use in the Unity game engine.