Yes, i believe they are now boyfriend/girlfriend, although they used to be just friends. In the episode "butt dialed" Mordecai accidentally butt dials Margaret while singing a song about her. it gets recorded on her voice mail. Margaret finds the voicemail, and she says its a good song. She and Mordecai talked about it and she set it to her personal calls for Mordecai, and tells him he better call her. Also, there is a spring break commercial with Mordecai holding hands with Margaret on a postcard with Mordecai's song playing in the background. They are definetely together now.
There are DEFINETLY just friends cuz in the episode camping can be cool, Rigby sed: "You probably shouldn't go, Margaret. I bet your boyfriend won't like you going on a camping trip with 2 dudes." Then she said: "What boyfriend? I'm not seeing anyone right now. Guys are jerks."
So, idk, there's probably NO chance of them ever going out(just saying)But they make the perfect friends :)
yes i believed that they are now boyfriend/girlfriend since start when they got camping,when mordecai send an embarass song on margaret phone ,when margaret tells to mordecai to make her a website,when mordecai is so depressed he though margaret is geting married ,when margaret birthday they dance ,when mordecai try to fix margaret diary when him and rigby want to impressed margaret and eileen and hi almost give margaret a kiss and hug her and lastly him and Margaret kiss but margaret said his breath is bad and go back in time due to him and margaret not to kiss surely mordecai and margaret are now BF/GF.