Both Kasparov and Karpov at the peak of their careers are just amateurs in terms of their performance against their peers compared to Fischer.
Neither Kasparov nor Karpov even had the ability to win a match against a top ranked contender with a score of 6-0 (against Larsen and Taimanov) and beating Petrosian in 9 games with a margin of 4 wins.
Neither of the two could have dominating performance against their peers as Fischer who finished Palma de Mallorca 3.5 points ahead of Larsen, Geller, Hubner Taimanov, Uhlmann, Portisch, etc.
Kasparov and Karpov could never have dominated a field the way Fischer dominated Palma de Mallorca? Kasparov and Karpov have won dozens of much stronger tournaments than that. To begin with Palma didn't have Spassky and Korchnoi, the two strongest Russian players. Fischer scored great results against the bottom half (but lost against Larsen). Fischer played four stronger tournaments in his career and didn't win any of them.
Kasparov rarely played in tournaments with so many weak opponents as Palma had (Jimenez, Rubinetti, Ujtumen, Naranja, Addison, Minic, Suttles, etc). Tilburg 1989 was a bit of an exception, where he too won with a margin of 3.5 points (in only 14 games instead of 23). The weakest player in the field was Piket, winner of Dortmund a few years later, all other participants were Candidates level. But this is just one of Kasparov's tournaments that rarely is noticed since he won so many stronger events for so many years, for example Linares 1999 with a margin of 2.5 points (14 games) ahead of Anand, Kramnik, Leko, Topalov, Ivanchuk, Adams and Svidler. All of the opponents being serious World Champion candidates.
Fischer's results against Taimanov and Larsen are often repeated, and one could just as well repeat that Steinitz won with a bigger score (7-0) against a much stronger opponent (world #2 Blackburne). Does that make Stenitz greater than Kasparov or Lasker? I don't think so, matches are a bit special. When Spassky played Larsen he too won his first three games, and then took it easy. Fischer himself said that his match against Taimanov just as well could have finished 3.5-2.5, but in matches the final result doesn't matter, it is only a question of winning or losing the match. As Larsen Taimanov threw away certain draws to try to win and it backfired. Anyway, beating Karpov time and again is always more impressive than beating Taimanov or Larsen, it doesn't have to do with match scores. Salov won a Candidates match 5-1 against Khalifman (later FIDE World Champion), and I wonder if anyone ever would mention that match if it had been 6-0 instead.
In the title match Fischer won 12.5-8.5 against Spassky, also there the result matters little, it was the same score Tal won with against Botvinnik. What mattered was that Fischer won and was the best player in the world in 1972. Since he wasn't interested in continuing playing it's hard to say how he would have fared against the stronger Karpov, who won 7-4 against Spassky in 1974. It would probably have been a tough match, but Fischer was at his peak and Karpov was still very young. At the same time Fischer hadn't played chess for years while Karpov was improving steadily.
He won the 1963–64 U.S. Championship 11–0, the only perfect score in the history of the tournament. In the early 1970s he became one of the most dominant players in modern history—winning the 1970 Interzonal by a record 3½-point margin and winning 20 consecutive games, including two unprecedented 6–0 sweeps in the Candidates Matches. According to research by Jeff Sonas, in 1971 Fischer had separated himself from the rest of the world by a larger margin of playing skill than any player since the 1870s