Quick explanation, then some observations below - I chose not to normalize over number of appearances so that I could just get "who did the most damage" onto what I find to be a fascinating list down nightmare lane. As you will see this incorporates a healthy mix of fantastic flame outs with guys who were just moderately bad but throughout an entire season. When I first started researching this list, I was worried it would be dominated by the fantastic flame outs - guys like Steve Trout - but the more data I collected, the more varied it got. I was also worried that I would get an inordinate amount of players from the early expansion years, but was "pleasantly" surprised to see a fairly even distribution of players across M's history.
I chose Baseball Reference, because I'm lazy. I'm sure there are better WAR numbers out there, but I imagine these have to at least correlate with how bad these past Mariners were.
On to the list:
Catcher: Scott Bradley 1991, -1.5 ("defense")
Dishonorable Mention: Miguel Olivo 2005, -1 (thought he'd be worse)
Jerry Narron 1981, -1.2 (who? oh, I thought his name looked familiar - why?)
First Base: Dan Meyer 1978, -2.8 (epic)
Dishonorable Mention: Richie Sexson 2007, -1.1 (year of the batting average beer price special)
Second Base: Harold Reynolds 1986, -1.2 (hey, he was young)
Dishonorable Mention: Bret Boone 2005, -1 (I remember he cried whenever he committed an error)
Shortstop: Mario Mendoza 1979, -1.6 (yes, he played for the Mariners)
Dishonorable Mention: Rey Quinones 1986, -1.5 (in only 36 games)
Third Base: Darnell Coles 1984 -1.7 (only 43 games)
Dishonorable Mention: Jeff Cirillo 2003, -1.4 (a full season of this guy)
Utility Infielder: Felix Fermin 1995, -1.7 (traded Omar Vizquel for this guy)
Dishonorable mention: Mackey Sasser 1993, -1.5 (I remember having this guy's rookie cards)
Outfield: Jeffrey Leonard 1990, -1.8 (scored 39 runs in an almost full season)
John Moses 1983, -1.6 (the worst of his numerous bad years)
Greg Briley 1991, -1.8 (I had no idea his defense was that bad)
Dishonorable Mention: Mike Morse 2013, -1.3 (John Jaso +1.1)
Designated Hitter: Alvin Davis 1991, -1.8 (Mr. Mariner twilight in the Mariners first season over .500)
Dishonorable Mention: Willie Horton 1980, -1.1 (another twilight season, doesn't seem fair to pick on these guys...)
A boo-worthy mention: Eduardo Perez 2006, -0.9 (a bavasi special)
Ryan Rowland-Smith 2010, -2.6 (awwww.....)
Mike Parrott 1980, -2.4 (classic Mariner)
Dick Pole 1978, -2.2 (Name of the Year)
Miguel Batista 2008, -2 (why?!?)
Joel Pineiro 2006, -2 (such promise....)
Dishonorable Mention: Carlos Silva 2008, -1.9 (hope we learned our lesson)
Greg Hibbard 1994, -1.6 (the Carlos Silva of 94)
Relief Pitchers - the cream of bad rises to the top. I remember 97 and 98*, and I imagine M's fans have all said the same thing when they saw any of these guys get called from the pen:
Byron McLaughlin 1980, -2.8
Norm Charlton 1997, -2.5
Bobby Ayala 1998, -2.1
Lee Guetterman 1986, -1.9
Dave Beard 1984, -1.7
*If it weren't for historically bad bullpens, Fassero would've had elite-ish wins and ERA.
What's surprising about the names? The lack of our usual whipping boys like Chone Figgins and WFB. Yes they were bad, but maybe not as bad as we think they were...... Our collective hatred of guys like this has more to do with context than anything else.
Speaking of context, there are several types of guys in here: young guys probably called up too early like Harold Reynolds or Darnell Coles. older guys who fell off a cliff, and had a good season the year before, which is probably why they got so much playing time - like Alvin Davis or Bret Boone. Then there are our "villains" - guys who were signed to big contracts or traded for useful players - like Carlos Silva or Mike Morse. I'm sure there were guys who were playing injured that had an uncharacteristically bad year too, but I'm not sure who exactly.