This is a question best asked of the MonoGame community, as they would know where you can get importers for TMX for it. The community here is spread between many different engines, so the chances of running into someone who uses both MonoGame and Tiled are lower here than in a community dedicated to the framework. This forum is best for questions relating to the Tiled Editor, and questions relating to using TMX files in various engines are best asked in those engines’ communities.
A couple of parsers that work with MonoGame are listed in the docs. One of the entries is MonoGame.Extended which adds direct Tiled support and I saw some recommendations for it in a cursory web search.