Pointing out win percentage is pointless when Hewitt played like 10 years after his prime. That plus nobody will check their win percentage except the biggest diehards or stats nerds. The only people whose win percentage get examined are the GOATS or GOATS on a specific surface, so nobody will look up the win percentages of Hewitt and Lendl on grass.
Novotna's win percentage is only 3% or something better than Seles on grass, so by this logic Novotna is only a slightly better grass courter than Seles. Federer has something like a 9% better win percentage on clay than Kuerten, do you consider Federer the better (or for a good laugh the much better) grass courter?
Edberg was not in his prime in 87. His prime was clearly 88-92, as
@helterskelter who is a long time fan(atic) of Edberg would vouch for I am sure. In 86-87 he was still a good player but didnt have the mental toughness to win big matches or big events (the Australian wasnt really a big event yet, but I am sure glad he won it twice when it wasnt anyway to compensate for his bad luck there later). I would agree that was a better win than any Hewitt had there though, but it isnt Hewitt's fault he got a super easy draw which he clearly didnt need in 2002, and the first good player he ran into both 2004 and 2005 was the one guy he couldnt beat- Federer. Lendl sure as heck wouldnt in a million years come close to beating Fed on grass either, I would be amazed if he could even get a set (which atleast Hewitt can do). Had Hewitt played say Agassi at Wimbledon 2002 he likely would have won, and that would be a bigger win than pre prime Edberg, but unfortunately in that respect Agassi chose to take a bad loss early rather than go on to his expected finals or semis loss to Hewitt. Had Hewitt played Roddick at Wimbledons 2004 and 2005 he would have won in 2005, and might have won in 2004 (I would guess Roddick but almost a toss up) and those would be huge wins, probably bigger than pre prime Edberg too, but he didnt get the chance.
Lets say Lendls prime on grass was 2002-2005 instead, we know he wouldnt have done any better than Hewitt in those years apart from 2003 probably. Best case in 2002 he wins with the same easy draw Hewitt had, but given his blowout loss to Cash in 87 dont be surprised if Henman with his serve and volley game beats him either. 2004 and 2005 obviously best case is he loses to Federer at the same stage Hewitt does, it would be comical to suggest Lendl beating Fed either of those years. Have Hewitt switch places with Lendl and I believe he still wins a Wimbledon, maybe more than one. I could definitely see him beating Cash in 87. It wouldnt be easy at all since Cash was in great form and played tennis worthy of being the Wimbledon Champ, but Hewitt is excellent at countering that kind of player. He was a nightmare match up for Rafter who is probably better than Cash, and extremely similar in playing styles/strengths in every respect. I could see Hewitt having a shot at Wimbledon 89 and 90 if he played Becker in the semis in 89 and 90 (he would have no chance vs final version Becker of 89 and semi version of Edberg in 90, but if he played Becker 89 in the semis instead and Edberg in 90 in the final in good but less ominous form he definitely has a shot).