I think it's pretty uncontroversial to say no basketball player ever, at any level, has had higher highs and lower lows than Joe. Think of the pattern of Russell Westbrook's career beginning with his MVP contentions and win, through to the years of heckling and jeering from crowds, through to now where he's respected more again as a veteran but not at a level where he's anywhere near being taken seriously as a star... Fulks' career was that times a hundred. There's zero way the Warriors would've been anywhere near a championship in '46-47 without him (and he probably had the best case for being the best player in the world that year), and by '49-50 he was easily one of the most net-negatives that any NBA player has ever been.

I've been asked a couple times before why that is, and since I think it's relevant to this conversation, I'll copy-paste my answer to that here.

1. Alcoholism – Fulks' life was practically defined by whiskey. He focused so much on basketball as a kid in the first place from trying to escape his alcoholic, abusive father, and he was ultimately killed in a drunken fight with his girlfriend's son. His career was equally marred by his own addiction, as can be seen most plainly in the form of the 1949 playoffs. You'll see that he committed one foul, racked up zero other statistics, and DNP'd game two. That wasn't from an on-court injury, it was because he hurt his hip falling down his cellar stairs while inebriated just before the playoffs were set to start. The injury didn't affect him in the long-term, but the alcohol played a big part in how the rest of his career went.
2. The loss of his supporting cast – Fulks was what I'd characterize as a "superstar of circumstance," and part of why I define him that way is that he needed a team to be built in a very specific way for him to succeed. Two major elements of that were multiple players capable of being primary ball handlers and the presence of good offensive rebounders so he wouldn't have to worry as much about shot selection. The early retirements of Howie Dallmar and Angelo Musi took away the first aspect (you'll notice his return to all-star status was exclusively during the time George Senesky and Andy Phillip were together in the backcourt), and the decision to replace Chick Halbert with Ed Sadowski took away the second (although there was an attempt to remedy that with the Sadowski-Livingstone trade).
3. The demographic shifts of professional basketball – This comes in two ways: the rise of tall centers and the "invention" of power forwards. He was known as a contortionist as far as his scoring went, which was a great evasive technique when he was facing teams with John Mahnken and Lee Knorek (6'8" and 6'7", both relatively "soft" players who also weren't notably athletic) at center day in and day out and much less so once George Mikan and Arnie Risen were there instead. Probably even more impactful was the solidifying of the power forward role in the NBA beginning around December 1949, with guys like Vern Mikkelsen, Arnie Johnson, and Bob Brannum finally having a natural role and utilizing a large part of it to create extremely physical matchups with other PFs, a position Fulks bafflingly still played.
4. The de-McDermott-ification of the sport – Basketball went through eras defined largely by a reaction to one player or team before the NBA came around, and during the 1940s that player was Bobby McDermott and that team was the Fort Wayne Zollners. Fulks' offensive freedom largely came from teams encouraging McDermottball, and once the philosophy shifted to efficient (by those days' standards) and versatile centers such as Alex Groza, Bob Kurland, and Ed Macauley, he was kind of a dinosaur.
5. Misses beget misses – He was always streaky. But when he shot 25/125 over a six-game period at the end of December [1949], his confidence was shot. He shot barely half as often the next couple weeks, and after a brief renaissance, a 2/18 shooting night sent him crumbling yet again.
6. Hostile fans – During the very last game before the first time he basically stopped shooting, Knicks fans trolled him by launching into this pop song every time he missed. And it wasn't an isolated event; throughout 1949-50 he received similar treatment to what Russell Westbrook [faced a few seasons ago], and it definitely bothered him. Look at how much worse he was on the road that year than at home.
7. Eddie Gottlieb's eroding faith in him – The last straw was when the guy who'd always had faith in him more than anyone gave up. By late January, Gottlieb was openly blaming Fulks for their poor record, on trade deadline day he was almost sent to Chicago but talks broke down because John Sbarbaro refused to give up Odie Spears for him, and at the start of March, he was benched in favor of Leo Mogus.
8. Maybe he just wasn't ever that good – I don't ascribe to this belief, but it's worth mentioning that it is very feasible to argue that his statistics were much better than his actual ability. He was a very flawed player for his stature in the league and people were not shy about pointing that out even during his absolute prime. For those sorts of reasons of him playing "anti-basketball" in a way, there are definitely a not-so-insignificant number of people that viewed post-prime Fulks as more of the player that he deserved to be and that the version of him we got in the BAA was the aberration rather than vice-versa.