I'd argue that Oettinger is playing worse against Edmonton than Hellebuyck played against Dallas, not that Oettinger was outstanding prior to this series either. He was passable and probably stole a game. Hellebuyck had a couple of meh games but managed to shut the door very well in 3 of them, and 5/13 goals against were scored on the penalty kill. Given what we've seen from other goalies these playoffs, I really don't think Hellebuyck's performance has been historically bad. Freddie is getting cooked (5.33 GAA this series?), Otter is being beat clean on 5v5 rushes and is giving up shots he should absolutely save, letting in 3+ per game. The Oilers have been stingy on defense and aren't giving up much - it's a major uphill battle to score 4 on them and Otter is not giving Dallas a good chance to win.
Hellebuyck vs Dallas:
Game 1: 3 GA, 0.875 SV% - 1 PPG
Game 2: 0 GA, 1.000 SV%
Game 3: 5 GA, 0.807% - 1 PPG
Game 4: 3 GA, 0.906% - 2 PPG
Game 5: 0 GA, 1.000 SV%
Game 6: 2 GA, 0.900 SV% - 1 PPG
GAA: 2.16
Oettinger vs Edmonton:
Game 1: 3 GA, 0.889 SV%
Game 2: 3 GA, 0.880 SV%
Game 3: 6 GA, 0.778 SV%
GAA: 4.00
Teams' defensive structures vary widely and match up well against some strategies that work on others. LA's defensive structure was excellent this year but Edmonton knows the way around it. It's possible that another team could've beaten Edmonton, which is the beauty of the playoffs. Clearly Edmonton knows how to skate circles around Dallas' structure, but Oettinger can't bail them out.