Character spoilers for Avengers: Endgame in this post. (Also spoilers from 2006 for Battlestar Galactica)
On top of all of its other problems, the “fat Thor” plotline in Avengers: Endgame is a retread of Battlestar: Galactica in 2006, when they similarly showed the impact of a time jump by putting Lee Adama in a fat suit as shorthand for “he got depressed and gave up.”
Like Chris Hemsworth, Jamie Bamber (the actor who played Lee) had been framed in the series up to that point as “the hot one*,” with lovingly featured biceps and six-pack. In both cases, the framing was presumably supposed to add a little extra jolt to the physical change and the time-jump reveal: look, he didn’t just go from average to fat, he went from ripped to fat! Extra! Extreme! Hilarious!
Hemsworth hasn’t said anything publicly about the character’s physical presentation; Bamber similarly never offered his thoughts at the time on his character’s presentation or how the audience reacted to it. (Given the nature of ongoing sci-fi fan conventions, he may have spoken about it later.) That reaction was fairly vicious, as the show had struggled to find compelling storylines for Lee at that point in the narrative and often used him in an antagonist position, priming the audience to react to his reveal after the time-jump with derision.
In contrast, public pushback to Endgame’s Thor presentation started after the premiere showings, with PSA tweets going out on Twitter and notes on other platforms warning fat audience members that there was an (unnamed) character’s weight gain presented for laughs. In the days after the film’s wide release, more specific thinkpieces and reactions were posted on multiple sites, as well as additional and more extensive Twitter discussions. The response was very different from that to Battlestar’s reveal, and demonstrated how fat politics has grown as a movement and how awareness has deepened at least in some sections of the sci-fi fandom internet; a small positive to take from a disappointing decision on the filmmakers’ part.
Ultimately, this choice of character presentation (calling it character development would be inaccurate) was lazy shorthand when BSG did it, and it’s lazy shorthand now. Depression, PTSD, exhaustion from loss and endless war—these can all cause physical changes in the body. Exploring those as a plotline would be well worth it. Making them a punchline is not at all worth it, and the science fiction genre shows little inclination to change their ways in this regard.
* I am aware that all of the Avengers are hot, but the rest of them have something else going on instead of having the hotness foregrounded solo; I would argue that Thor does not.