My MMR is probably close to accurate, I feel as a player I'm borderline Platinum/Diamond league player. I'm currently sitting in the middle of Platinum, though if the MMR history is correct, it had me at 1700 one week and 2600 the next. Though I have a friend who only plays hero league with me or one other person and he has a pretty high rating. So if I ever play with them I assume its skewing my MMR rating a bit.
Personally I don't care about MMR, I know my limits and I know I can't cleanse a Stitches hook on Uther, but I also know my map awareness and decision making are top notch through 10 years of MOBA experience. If you're worried about MMR, stop and just play. If you're able to get Diamond league actually solo queuing, then you may be a competitive gamer, but I can tell you that very few of the current HotS "pros" have done that because they're too busy queuing with 3-4 other people everyday.
Edit: They just need to make the game free, get more people in the community, fix their matchmaking (please God please), and add the team league so groups will fuck off ranked play.