Are you sure about the 2015 bit, bud? I didn't, and don't follow the Bundesliga that closely, so can't comment on my own views but from what I read on here, his stats for the season and also being voted into the Bundesliga team of the season (or a player of year nomination or something along those lines) - I'd say it's quite possible that Mkhi was one of their best players, if not their best player, in 2015/2016 season.
What you say is correct, concerning team of the season, player of the season etc. in his last year for Dortmund.
However, his stats are misleading. Dortmund had to qualify for the Europa League that year and Mkhitaryan amassed goals against 3rd grade opposition in the qualifying rounds.
I am judging him over the 3 years he played for Dortmund. If there was a "worst player of the league award" in the Bundesliga, Mkhitaryan surely would have been awarded with that in his second season for Dortmund. His first season was o.k., but he single-handedly prevented Dortmund's progress to the CL semi-finals with two god-awful performances against Madrid that year.
Even in his good third season he went missing in the cup final against Bayern and refused to take a penalty in the shoot-out, forcing defenders to take and miss them.
I am not denying that his last season for Dortmund was good and he often looked brilliant. The talent is clearly there.
But to claim that he was Dortmund's best player is just ridiculous if you look at his team mates like Reus, Auba, Gündogan etc. who performed on a world-class level on a weekly basis while Mkhitaryan was world-class one week and looked like a sunday league player the next.
edit: to keep this on topic: I don't think there is one BVB fan who would like to swap Mkhitaryan for Pulisic right now.