What makes a trade terrible?
The players your team gave away doing much better than the players you received, sure, but that's bad luck sometimes which is nothing special for the Mariners. A terrible trade needs to have bad process and to leave scars. Whether that's from dealing away a fan favorite, or leaving a black hole in the lineup, a bad trade makes you confused when it happens and bitter for years after. I believe the scars of the three way trade in January 2013 between the Mariners, Athletics, and Nationals that sent John Jaso to the A's for Mike Morse are still being felt today.
The Mariners haven't had a whole lot to show for the past 16 years, so the small victories we get mean that much more. On August 15, 2012, Felix Hernandez and the Mariners did something only 22 other pitchers and teams had accomplished in MLB history by pitching a perfect game. Baseball is full of trivial historical records like the one Raul Ibañez would break the next year, but perfect games are undisputedly important. He joined legends like Cy Young, Sandy Koufax, and Phillip Humber, and no one has accomplished that feat in the five seasons since. John Jaso caught that game. He was also pretty good at baseball that year, putting up a .276/.394/.456 slash line in 361 PAs and finishing the season with more strikeouts than walks, good for 2.6 fWAR.
Mike Morse had a breakout year in 2011, and put up a .291/.321/.470 in an injury shortened 2012. His defense was bad, but power like that was much rarer in 2013. After trading away Ichiro in 2012, the Mariners outfield was Michael Saunders, a slightly broken Franklin Gutierrez, and other Bad Trade Guy Casper Wells. However, the Mariners were not close to competing in 2013. Trading away a player worth 2.6 WAR with three years of team control for one year of Mike Morse did not fit with the timing of waiting for the pitching prospect cerberus of Hultzen, Walker, and Paxton to arrive. But Morse hit a six home runs in his first nine games of the 2013 season and Casper Wells was placed on waivers.
Morse would go on to hit seven more home runs in his remaining 67 games as a Mariner before being traded to the Orioles for cash considerations. His outfield defense was brutal and he was worth -0.7 fWAR. He was a reminder that the sabermetircally inclined Jack Zdurencik was now ignoring defense and OBP and prioritizing right-handed sluggers in a park that suppressed them. Moving Jaso meant that full time catching duties would go to Jesus Montero, who played more games at DH than catcher because he was a worse at catching than both Jaso and a 33 year old Miguel Olivo. Veteran Kelly Shoppach was brought in to be the backup in what would be his final season. It did not go well for either of them. Mike Zunino was rushed to the majors after fewer than 100 minor league games, and his plate discipline was stunted. Humberto Quintero and 41 year old Henry Blanco started multiple games. The A's didn't really need Jaso, as Steven Vogt and Derek Norris both emerged as catchers with stronger defensive skills, but Jaso would put up solid rate stats before being traded to the Rays with Boog Powell for Ben Zobrist. He would go on to become a white guy with dreadlocks on the Pirates in his free agency, and no amount of good judgement at the plate can make up for that bad judgement of cultural appropriation. His batting line during his remaining years of team control was .267/.358/.407 with a 122 wrc+ and a 17.1 K% and 11.1 BB% and he was worth 3.6 WAR in only 809 PA.
During Jaso's remaining years of team control from 2013-2015, Mariners catchers put up the following stats:
.187/.243/.317, 57 wrc+, with a 28.6% K%, 5.5 BB% and 0.0 fWAR in 1830 plate appearances.
1055 of those PAs were Mike Zunino, who is Good now. It took him a long time to become Good. During those three seasons, Zunino could have developed the plate discipline he has shown recently away from the media. He looked lost at the plate, but remained in the Majors because they had no other promising players at catcher. Even though both Jaso and Morse retired this past offseason, the pain of those 1000 plate appearances for Mike Zunino remain. We would not need to remind ourselves that Mike Zunino is Good, because we would not have seen him struggle for 1000 plate appearances. A player who was a big part of a rare bright spot in Mariners history would have grown with the team. John Buck wouldn't have been cut on his birthday in 2014, and his disappointing stats would have been replaced with a full season of Jaso's for a team that was a game short of the playoffs.
Jason Varitek or Adam Jones were better than John Jaso, Luiz Gohara and Chris Taylor might be as well, but no trade left a bigger void for less hope.