I always thought that his 3-0 record against Borg on har-tru from 1974-1976 (all 3 of those wins came on the big stage), was a very big deal and hugely impressive.
I previously calculated Borg had a 122-15 W/L record on clay from 1974-1976 (obviously records including surface details are likely not great from that era so that probably won't be fully accurate), and Connors inflicted 20% of those defeats and more than anyone else. The only other players with multiple victories against him during that period were Panatta and Ramirez with 2 each.
Excluding Connors, he was 35-7 (83.3% success rate) against his main rivals on the surface during that period:
Orantes 6-1 (1-0 on har-tru)
Panatta 5-2 (1-0 on har-tru)
Vilas 5-1 (2-1 on har-tru)
Nastase 2-1 (1-0 on har-tru, or 2-2 overall 1-1 on har-tru if we count their World Invitational Tennis Classic match in Hilton Head in 1975 as the ATP do, though IIRC Borg also had wins in invitational events on clay during that period).
Kodes 3-0 (1-0 on har-tru)
Ramirez 3-2 (2-0 on har-tru)
Solomon 4-0 (2-0 on har-tru)
Dibbs 7-0 (4-0 on har-tru)
Kodes was the only one of those 8 players that Bprg never beat / faced in a clay court major during that period.
I will say that I think you can argue that Connors' ban from playing at RG in 1974, as ridiculous and as unjustified as it was (he was in Paris, wanting and ready to play), actually improved his Wimbledon title chances, given that there was only 1 week in-between RG and Wimbledon that year, and he played and won a grass court tune-up event in Manchester during the 2nd week of RG.
The Connors 'never won a title on red clay' shtick is both false as his 3 titles in North Conway were on red clay and not har-tru (due to heavy rainfall the 1976 final was played nearly 100 miles away on indoors, although he had already won 5 matches to get that far), and overblown given that it was not like he entered masses of tournaments on clay in Europe during his career.