Actually yes, Ray Bourque was not a great playoff performer. Go look at the Bill Lindsay goal, the Brad May goal, Bourque burned for goals by mediocre players. He was lucky he was able to ride on Joe Sakics coat tails.....
Right, because getting burned once or twice (something that happens to every player, no matter how good) is what matters, not high end performances and carrying your team as far as it gets.
Bourque did the latter a ton, about as much as a non-goalie possibily could. Do you judge Patrick Roy based on his three Conn Smythe Trophies and multiple great playoffs runs, or the "statue of liberty" and following that getting torched by Detroit in game 7?
Greatness doesn't mean that you are flawless.
Will winning a Cup or not make a difference in perception? Yes.
Should it? No, absolutely not. It's people ignoring that one player cannot ever win on his own, and there are way too many of those. A team winning has no bearing on how well the player performed. Someone doesn't become a better alltime player just because he lucked himself into becoming a Cup-winner as a fading 4th liner when he got old.
Joe Thornton gets a lower standing because he got the reputation of a playoff-choker, not because he didn't win. He never quite managed to shake that label, though he did have quite a few good runs as well. You fail once or twice, even if it happened due to injury, and it takes a lot for people to forget about that.