Jeez, What happened?
Seriously, "two thumbs down?" "worst movie ever?" are we watching the same film?
My theories:
1. It was a total financial disaster ($62m in losses). Maybe the one of the biggest flops to date in 1997.
As always happens, people who don't really watch a lot of films but like to sound like they're in the know, conflated "[one of the] worst [financially perfoming] movies ever" with "worst movie ever." Just because no one watches something, doesn't make it bad.
2. Critics hated it because Dances with Wolves raised the bar for Kevin Costner so high, and Waterworld was an artistic and financial disappointment. Following up with another post apocalyptic movie, and one that had some minor flaws, wasn't enough to dig him out of his hole and they pounced. Nothing but another Dances with Wolves would have saved him at that point. Ironically, Postman was trashed alot more than waterworld, yet was a much better film.
3. Kevin Costner overstaying his welcome. Kevin Costner's movies (both starred in and directed) were a weird range of outstanding to pretty bad. So he was a mixed bag, who was still surrounded by a lot of residual glowing praise. That usually reads as hype. Critics were thrilled to deflate his mythos and "The Postman" was the unfortunate victim. As usual, the public bought the hype and stayed away in droves.
This movie was thoroughly enjoyable for me. I see it every few years when I get on a post-apoc trip and want to see if I'll "come to my senses" and suddenly hate it like everyone else seems to. Minor flaws aside (including the atrocious "everything is back to normal" costuming in the statue-dedication scene), it has aged well and is IMO stunningly underrated.