Honestly, BJ had a very hard fall. He lost the last 7 fights of his career. Before that, he was 16-7-2, which at 1st glance is hardly impressive. But context matters. Those 7 losses before he fell off a cliff:
Pulver - yes, BJ was favoured but Pulver was very highly regarded at the time, Pulver was actually the champ going into the fight. BJ later avenged this
Machida - would later become UFC LHW champ, a guy who was several weight classes bigger than BJ. Before you claim Machida wasn't very good back then, Machida was 5-0 at the time, & had already TKO'd both Bonnar & Franklin (future UFC MW champ). No LW could reasonably be expected to beat Machida, being realistic
GSP - IMO, the GOAT. Yes, GSP was not yet the GOAT, but he was already seen as a future champ & had already fought for the belt (but lost to Hughes in the 1st fight). GSP was the #1 ranked challenger going in, BJ was coming off a UFC hiatus. BJ lost a close, competitive split dec, I do think GSP rightfully got the nod, but how many other LW could've done that? Khabib wanted 40 yr old GSP to cut to 155 because it wasn't fair, yet BJ did it & almost pulled it off
Hughes - this was a fight that everyone thought BJ was winning, until he wasn't. It's no shame to lose to Hughes either, Hughes has the 2nd most title defenses in UFC WW history, an all time great. Also, BJ is actually 2-1 vs Hughes. Again, out of his weight class but BJ had a winning record vs an all time great WW champ
GSP 2 - yup, this fight was very 1-sided after the 1st rd (which GSP also won, but neither guy did anything in the 1st rd). Complete domination by GSP. Nothing much to say other than GSP was in the midst of a dominant stretch where he was seen as having no real challengers at WW, & since BJ put on a heck of a performance in the 1st fight, & was the dominant LW champ, they decided to redo it
Edgar 1 - to this day, I maintain BJ won the 1st fight. To show that I'm not crazy, MMA Decisions showed that 8 MMA media scored it for BJ, only 1 for Edgar. All 3 Sherdog writers scored it for BJ, & most fans did as well
Edgar 2 - yea, Edgar was totally the better fighter here. This is when I thought damn, Edgar is actually just too quick for him, & BJ was a guy known for his explosiveness. Edgar is an all time great LW
Uno (draw) - this was a lacklustre title fight & the UFC closed down the division after this. I think BJ was unmotivated (LOL), because he previously already easily destroyed Uno, whom was always in the title mix in those days
Fitch (draw) - Fitch was widely regarded as the 2nd best WW in the GSP era. He was consistently ranked #2 for a long, long time. It's weird to me that people hold it against BJ, imagine if Khabib went up to WW & got a draw vs Colby or Khamzat, would that be considered a failure?