How did this group start?
This group started thanks to the previous founder . She, along with and were the original founder and co-founders to this group.
How did this pairing start?
The current founder, did a crossover story a few years back that had interactions between Mordecai and Twilight Sparkle. People that read the chapters liked the interaction between them, and a liking to this pairing grew. She did a few artwork for the pair, whether it was to cover a scene from her story, or a different kind of project she was doing that mentioned how much people liked the idea of Mordecai being paired with Twilight. These are the following links to the story that introduced the pair:
Doesn't Mordecai hate unicorns? Why would he like Twilight?
We are aware of the "Unicorns Have Got To Go" episode on Regular Show, but last I checked, Twilight isn't like those Fraticorns, (Frat boy Unicorns). And in all honesty, would you really hate this face?
Doesn't it seem weird that your pairing a bird with a horse?
There has been stranger pairings out there, whether it's related to My Little Pony, or not. I don't know how this one's the weirdest.
You do know that both Twilight and Mordecai have their own love interests, right?
Yes, we are aware that Twilight had a crush on Flash Sentry from the Equestria Girls movie, and we know of Mordecai's relationship with Margaret, and his, soon to be, new girlfriend in CJ. This group is for fun, and if you take the shows that seriously just to bash this group, I'm sorry for you.
Why would Twilight want to date a total slacker like Mordecai?
True, Mordecai is a slacker, but he is still responsible, he risked his own life to get the last of the chairs so that he could prove he wasn't a slacker. If you want to talk about a total slacker, his name is Rigby; I mean come on, the guy hired someone to do his jobs so he could slack off. As for Twilight, after that whole "Lesson Zero" incident, I would think she would NEED a guy like Mordecai. It's one thing to be great at what you do, but you can't take the littlest things so seriously, like missing a deadline for a letter that Celestia doesn't need. He would be the kind of guy to balance her out, just like she would be the kind of girl to balance him out.