It is shocking to me that a QB that had all of one year under his belt for UNC was drafted 8 slots ahead of Mahomes. The only 2 negatives I could see with Mahomes' college stats are that (a) he threw a fair number of picks; and (b) his rushing average wasn't all that great (2.2 ypc his final season). 2 running backs and a safety were taken ahead of both Mahomes and DeShaun Watson.
If you look at the teams that were drafting ahead of KC/Buffalo/Houston in that draft:
Cleveland: Had so many holes that I cannot fault them for picking the defensive star of that draft.
Bears: Picked Trubisky
49'ers: Colin Kaepernick had a subpar season, but hard to know what they were thinking, and ended up with JG.
Jax: Had recently drafted the immortal Blake Bortles with the #3 pick.
Titans: Had drafted Mariota 2 years previously.
Jets: Need I explain more?
Chargers: Were admittedly all set at QB, but still surprising that they didn't at least look into picking for the future with Rivers turning 35
Bengals: See NYJ above.
Saints*: Not really in the market for a QB at the time, so cannot blame them for passing on Watson, and Lattimore has at least made the Pro Bowl.
Bills at least got a lot of draft capital in return, and netted themselves an All-Pro CB and a solid starter at linebacker out of the deal as a result.
We should say that even at the time the general rection to Trubisky was "HUH?" Everyone expected Watson there though also, amusingly if I remember correctly it was seen as a weak QB class.
As to the teams I generally agree:
CLE: Garrett was a monster, and they knew they were tearing down another year so they expected a QB the next year (they got it in Baker)
49ers: Team was a mess and Kap was not far removed from being great... also took Beathard in the 3rd, defensible decision making
JAX: Bortles was actually decent before playing through a shoulder injury... though that they took Fournette at #3 also lets you know that they are terrible at drafting
TEN: Yep Mariota was always getting more time
NYJ: Had just used a 2nd on Hackenberg as well, but that's a pick you cut bait on if he couldn't even back-up as a rookie and you have a top 10 pick.
LAC: No reason to use a 1st, Rivers was a top QB and was likely to stay that way through most of any pick's rookie deal
Bengals: Pretty defensable, they had Dalton, who was above average and locking into a long-term deal. He was great 2 years before, and good despite a terrible O-line the year preceding the draft.
Saints: Yeah, they had a HOF QB still in the top 5 or so in the league.
So really I'd say the Jets and Bears, are the only really terrible calls there in terms of at the time knowledge.