As someone mentioned, Williams did a remarkable job with the 1969 Senators. But Washington was always a second-rate operation, and they reverted to form after that, then moved to Arlington. The early Rangers were a complete disaster all around, read Mike Shropshire's book (seriously, it is hilarious.) Williams did not fare well with the Rangers, bottoming out in 1972 at 54-100 (that was a strike-shortened season.) His replacement was Whitey Herzog, who couldn't even get through a full year with that traveling horseshit show (47-91).
Williams was often referred to as a terrific teacher of the art of hitting in his time, though. Tony Gwynn, who befriended Williams in his final years, sure thought so. Williams gave Gwynn, already a sure-fire HOFer, some advice on turning on the inside pitches that Gwynn swears was a revelation to him at the time, something he should have known all along or maybe did once but forgot. But Williams picked it out right away. On that 1969 Senators team, Williams had light-hitting SS Ed Brinkman (lifetime .224/.280/.300) hitting .266/.328/.325. LH power-hitting 1B Mike Epstein (lifetime .244/.358/.454) hit 30 HRs and .278/.414/.551, all career highs by far. 3B Ken McMullen (lifetime .248/.316/.383) went .272/.349/.425. And so on. All those guys credited Williams with working with them to improve their approach. Each was traded away a year or so later -- Washington was a foundering concern without an apparent cohesive plan, and consequently had a high roster turnover -- and each's offense fell off dramatically after getting away from Williams. Could be a coincidence, of course. But Brinkman for one specifically said that when he got to Detroit he fell back into bad habits without Williams harping on him every day, and his hitting dropped off significantly. Back before the Washington years, Williams had been an unofficial hitting coach with the Red Sox, mostly during spring training when he wasn't fishing. Yastrzemski has mentioned getting help from him, Rico Petrocelli said Williams had him turning on middle-in pitches, aiming for the Green Monster. Prior to that Petrocelli, then a SS, had been a spray hitter. After, he hit 40 HRs. Reggie Smith has mentioned getting help from Williams, as well.
Williams may not have been a good manager, but unlike Cooper apparently, he had a very clear idea about a proper approach to good hitting, was passionate about it to the end (ask Gwynn), and by many accounts was pretty good at getting it across to younger players. I don't think he is a fair comp to Cooper in that regard. To put it another way, I wish we could unfreeze Teddy Ballgame and have him talk to Pence about his approach, such as it is, at the plate. But would Pence listen?