Is Mitch Trubisky a bad quarterback? Let's look at some other QBs drafted (excluding Mahomes) in the 1st round from 2015-2018, and see their career numbers as compared to Mitch.
QB A: 75 games, 70 starts, 19,812 passing yards, 121 TD, 88 INT, 264.4 yds/game, 86.9 rating over career, 28-42-0 record as starter
QB B: 64 games, 61 starts, 13,433 passing yards, 77 TD, 45 INT, 209.9 yds/game, 85.9 rating over career, 29-32-0 record as starter
QB C: 68 games, 68 starts, 17,937 passing yards, 107 TD, 54 INT, 263.8 yds/game, 92 rating over career, 42-26-0 record as starter
QB D: 68 games, 68 starts, 16,811 passing yards, 113 TD, 50 INT, 247.2 yds/game, 89.2 rating over career, 35-32-1 record as starter
QB E: 52 games, 51 starts, 13,850 passing yards, 98 TD, 35 INT, 266.3 yds/game, 103.7 rating over career, 28-23-0 record as starter
QB F: 44 games, 43 starts, 10,634 passing yards, 74 TD, 43 INT, 241.7 yds/game, 89.7 rating over career, 22-21-0 record as starter
QB G: 36 games, 36 starts, 7,656 passing yards, 42 TD, 37 INT, 212.7 yds/game, 78.4 rating over career, 12-24-0 record as starter
QB H: 20 games, 16 starts, 2,845 passing yards, 12 TD, 19 INT, 142.3 yds/game, 63.5 rating over career, 3-13-0 record as starter
Mitch: 49 games, 48 starts, 10,092 passing yards, 62 TD, 35 INT, 206 yds/game, 87.1 rating over career, 28-20 record as starter
Really, the only QBs who are definitively better is QBs C, E, and F. The rest, you can make a good case that Trubisky has performed better. The numbers, I think point to Trubisky being a good quarterback, probably underappreciated due to his draft position (#2 overall), the fact he was traded up for (the Bears spent four picks to move up that one spot), and the phenomenal success of Mahomes. But in terms of being a bad quarterback... I don't think that is fair to say.
Of the nine non-Mahomes QBs picked in the first round from 2015-2018, Mitch's rating is right in the middle. When he starts, his team has generally won - in fact, he's arguably got the second-best win-loss record among all these QBs.
What do the Bears need to do to help him get better? Probably fortify the offensive line - to the tune of a new RT and an eventual replacement for Leno at LT. The draft has two guys who I think could do that in Jackson Carman of Clemson (a 6'6", 345-pounder) and Daniel Faalele of Minnesota (6'9", 400 pounds). Which ends up at RT and which is the eventual LT is hard to say, but both also look to be very good in the run-blocking department.
Draft a young bruiser at RG to solidify that group (Trey Smith could plug in as starter, with Daniels as reserve), or draft Josh Myers in the first round (have him back up C/G behind Daniels-Mustipher-Whitehair), and then I think the Bears are in shape. They probably need another good RB as well to make Trubisky really effective, but I don't think Trubisky is a bad quarterback, and the comps of first round picks from 2015-2018 bear that out.
By the way, here are the comps:
QB A is Jameis Winston - currently a backup for the Saints
QB B is Marcus Mariota - currently a backup for the Raiders
QB C is Jared Goff - Rams starter
QB D is Carson Wentz - Eagles starter on his way out due to injuries
QB E is Deshaun Watson - Texans starter
QB F is Baker Mayfield - Browns stater
QB G is Sam Darnold - Jets starter
QB H is Josh Rosen - Buccaneers backup