Mono's use was very limited because it was an out of date and slower implementation of .NET. It's used in such niche contexts (Xamarin and Unity) that it was easy to determine if the team you are applying is using MONO or Framework.

.NET core changed the ability to make the Windows assumption. Almost everyone has received the memo by now, but there was a period of time between .NET core 1.0 (June 2016) and the release of .NET core 2.0 (August 2017) where the platform was changing so much and the performance wasn't as good as framework that anyone without a simultaneous interest in Linux rightly ignored it. After the release of .NET core 2.0 the entire community began accepting it as a faster platform that runs comfortably on cheaper servers. A larger and larger portion of new projects have been running .net core. Now, with the planned release for .NET 5 next year .NET Framework is dead. However, if you were interviewing with a .NET team before June 2016 even asking the question "What operating system will this run on?" would have been a sign of incompetence. Of course you'll be building .exe and .dll files. Of course the webserver is going to be IIS. Those were the only things .NET was good at building.