Personally, I would not do this as an Avs fan because of the difference in Jost's play from this season as compared to last season.
A lot of people will look at his goals, assists, points and figure he's just never going to get there but you would think that fans of a team that just acquired a 24 yr old kid like Burakovsky would understand that development isn't linear.
I've always been a big Tyson Jost fan but this season in particular I find that he's more involved in the play. He's drawn a lot of penalties so far this year (9 - 3rd on team after Mack & Kadri), he takes hits to make plays (sometimes needlessly and he'll need to pick his spots more...if he wants to survive) and he's been noticeably good at timely takeaways that leads to scoring chances. Overall, I'd say he's been good offensively and defensively in the roles he's been asked to play this season.
I haven't seen much of Mittelstadt this season but unless you can say as much about other aspects of his game improving as I just have about Jost, then that's not a trade I would make.
It's too early to say for sure for either guy at 21-22 years old. To put things in perspective, it would be like Flyers fan wanting to trade Morgan Frost because he hasn't made the team yet. If either Mittelstadt or Jost had taken the slower development route, fans of both teams would be eagerly awaiting their arrival. Give it a couple of years still.