Yes and no. I wrote a few long posts about Ken's skill in the 90's,
like this one, and noted later in that same topic that being a roided-up 225 pounder, much larger and stronger than most of his opponents, and being a
decent wrestler was enough to have good success back then, despite terrible, piddly GnP and awful striking.
Now, when Ken came back in the early 2000s, his knees were shot and he couldn't wrestle well anymore, AND his cardio sucked. However, his striking had improved a lot, and he could actually throw some hard punches, even if his hand-speed and technique were still lacking.
Now, if Ken hadn't gone to pro wrestling in 1996, how would he have done?
Mark Coleman, who he wisely avoided, would have smashed Ken to pieces, better striking or not. Don Frye in his prime would have won a far more convincing victory over that version of Shamrock than he did in the early 2000s, when BOTH men had shot knees from pro wrestling and couldn't grapple, but Shamrock's striking had improved. Vitor Belfort would have certainly knocked Ken out quickly.
A Maurice Smith who learned takedown defense would have beaten Ken, too.
Now, I do think Ken would have beaten Tank Abbott and he would have probably won a boring decision against Marco Ruas. Gary Goodridge is a 50/50 match. Certainly, Goodridge can be beaten by lay-and-pray, which would be Ken's path to victory. However, Goodridge would also be the biggest, strongest guy with the best takedown defense that Ken would ever lay-and-pray if that were to occur.
In terms of the guys Ken actually faced from the 2000s onwards? Yeah, he probably wins a few of the fights he lost, like the one against Fujita, but comparing him to Couture is silly.
Couture was just a vastly better mixed martial artist in every regard. Way better wrestling, way better GnP, better striking (Couture's boxing was always underrated), and arguably better submissions, since Couture didn't give up dominant position to go for them the way that Ken did.