It was a forced change that was not justified. If the belief was that the Thor films needed to not necessarily be as dark as TDW going into film #3, that did not mean upend the entire tone and development of the character to turn him and his surroundings into comic relief. for Ragnarok's Thor being a goofy bumbler.
I did not say he was--the empty-headed, fat drunk was in Endgame, but the goofy bumbler started two films before that, which is a major change to what was once a character dealing with issues of his destiny / nobility issues--the very thing separating him (i.e., making him unique) from all other heroes in the MCU.
Picard believed he alone facilitated the slaughter of thousands of Starfleet lives while assimilated as Locutus, so the guilt and weight of it was close to unimaginable, but it did logically result in his extreme behavior seen in First Contact. For Thor, believing killing Thanos changed nothing was disheartening, but that should not have pushed him to being a sloppy, drunk bumbler / comic relief. He should have been at least as determined as the other Avengers to seek a way to reverse what had happened instead of suffering a character-killing blow that robbed him of his journey toward wisdom he struggled with in his earlier appearances.