I know people give you a hard time so I say this without any offense intended, but I know you're smarter than this.
First, it's as basic as watching the games - if you have then it's obvious how well Devon has played. He's been fantastic.
In terms of explaining why a player with a +WAR could be on a team with a losing record when he plays, this is also fairly obvious. He could hit 5HRs in a game, but the team will still lose if no one else gets on base and the Jays pitchers give up >5. Or maybe he makes a half dozen spectacular defensive plays that each save a run, but if the other team hits 5 HRs and the Jays score <5 runs, then it's a loss. So yes, luck is most definitely a factor. He's only 1 guy on a team of 25.