"Lambs" was a better film and it deserved Best Picture. Hopkins' performance was iconic. He deserved his Oscar. And if you want to get technical, DeNiro's performance wasn't even second best in 1991:

The other nominees: Warren Beatty for "Bugsy," Robin Williams for "The Fisher King," and Nick Nolte for "The Prince of Tides," were just as good as DeNiro that year. But none of them were as good as Hopkins.