Yep, I don't think there's been a worse thrashing. Connors was simply unstoppable in '74.
Bad result but Rosewall's 'thrashing' was mainly due to his 39 year old self not being unable to recover quickly enough after his long 4-set semi win over Newcombe, same as he couldn't recover after his 5-set win over Smith at Wimbledon that year. Little doubt Connors was going to win but the result was as much about his opponent's fatigue as his own level of his play. Same could be said about Graf's 6-0, 6-0 win over Zvereva. An absolute massacre and no doubt about the winner but it's fair to say the score was dictated by Zvereva's stage fright.
As far as I'm concerned, the absolute worst beatings in a GS final are;
1) Lew Hoad's 6-2, 6-1, 6-2 win over Ashley Cooper at the 1957 Wimbledon. I believe it was the quickest final since the challenge round was abolished and Hoad actually gave Cooper 1 game in the second set because he didn't want him to be embarrassed by losing a set to 0 during a Wimbledon final.
2) John McEnroe's absolute hammering of Jimmy Connors at the 1984 Wimbledon, 6-1, 6-1, 6-2. Connors was only 32 at the time, had won the US Open the year before, Wimbledon two years earlier and was the number 2-3 player in the world for the entire year. He was fortunate to win 4 games.
3) French Open 1978, Borg decimated Vilas, the reigning champion, 6-1, 6-1, 6-3.
4) Federer's win over Hewitt at the 2004 US Open. No excuses there, just one guy dominated the other.
5) Hewitt's win over Sampras at the 2001 US Open, 7-6, 6-1, 6-1. After the first set, which Hewitt never looked likely to drop, it was a slaughter.
6) 1989 Australian Open, Lendl bt Mecir 6-2, 6-2, 6-2. Didn't just beat him but completely broke his spirit.
A couple of other 'floggings' were
1) 1983 Wimbledon, McEnroe's 6-2, 6-2,6-2 win over Chris Lewis
2) 1977 French Open, Vilas beating Gottfried 6-0,6-3, 6-0
3) 2003 Aus Open, Agassi bt Schuettler 2, 2 and 1.
Although those were huge wins/losses I don't include them as the difference in standard between the two players was so great that the scores were relatively close to what you'd expect.