They went 73-9 in RS and 15-8 in the postseason. So yeah to say they drastically underperformed would be pretty accurate. But I don't think that's on the all stars.
That being said I don't think Klay or Dray were overrated. The Portland series was proof of that, despite Curry only playing 73 minutes Klay roasted then for 31 on 50/50/94 splits and Draymond absolutely tore them apart with 22/12/7/2/3. Draymond also absolutely carried in g7 of the Finals with 32/15/9 while Klay and Steph went for 31 on 36 shots combined. Klay was solid vs OKC, especially g6.
I think Curry can't avoid blame here. He was pretty meh in g5 . Not bad but you need more than 25 of 55%TS when the other team has two guys combining for 82. In g7 he was straight up bad, your MVP can't have more shots than points and 2 AST/4TOs. Especially when he didn't need to be great with Dray picking up the slack. He was great in the OKC comeback and I think he started running out of stamina, but they are champs if he's even close to his normal self in G7.
It's also fair to point out the peripheral players, usually you don't blame one or two guys underperforming because it's expected. But Barnes falling off an absolute cliff, Bogut getting injured in the Finals and exposing a very weak center rotation, Iggy hurting his back, etc. People mock this but it did matter.
Take Bogut, because he was out they filled in his g7 minutes with Ezeli (11 min, 0 pts, -9) and Varejao (9 minutes, 0 pts, -9). Bogut wasn't a star but he provided quality minutes and rim protection, and most importantly he didn't make stupid mistakes. Remember Ezeli jumping out of his shoes and fouling LeBron on a pump fake 3? This was a game where they were within 3 with a chance to tie, it doesn't take much to swing that.
So yeah, I wouldn't really say the cast was overrated in terms of talent and ability. Dray and Klay performed as you'd expect from the second and third options, they each had clutch games and kept the team dominant in the early rounds even when Curry was out. They were great because Curry being a god and their depth, both of which faded just enough in the finals for them to lose.