I think I manage to use Xtiled and convert it to monogame DLL. problem now is that even though I set the map file(.XNB) to build content and copy as always I always get the error on asset set as a non-content file.
EDIT:
Nevermind I got it working. I realise that content pipeline in XNA create a folder with the same name as the tmx file and inside of it is sort of index like
00.xnb
01.xnb
which I also have to add on the content project and set the build content and copy as always.
I dont know what that is but its a shame I have to go through that just to import it to monogame. I hope monogame team could atleast support tmx file on the pipeline tool.