There was a recent Skype interview of him posted here by the Amerks in house reporter. Skype isn't flattering to anybody but he still looks so pudgy and soft. Some of that is genetics but I think it has more to do with his commitment to physical conditioning/strengthening and diet. Sam Reinhart came back after his rookie season looking noticeably different, his neck was much thicker, shoulders bigger, less baby fat.
I think Mittelstadt has just had it too easy. That decision to stay in high school because he wanted to win a championship with his buddies really wasn't a good sign. Whenever he's interviewed about his struggles he always answers 'i'm not worried, it always has come around for me, i just have to be patient' - I think that is a very honest answer, he believes if he just keeps playing hockey everything will fall in place. NHL players are driven, they get stronger and faster, they hire nutritionists/chefs, hire skills tutors - he doesn't seem to get that part of it yet, it should be obvious to him by now what he has to do to be an NHL player.
He strikes me as a kid who if he doesn't succeed in the NHL it won't bother him, he'll be back in upper middle class Eden Prairie and with his connections and fame he'll get into some business and live a good life.
He's similar to Alex Nylander, just don't think either of them completely gets it.