It makes sense to follow up Thor's arc in Ragnarok/Infinity War by having him hit harder than anyone by the snap. The guy lost his father, his planet, his best friend and his brother in a matter of days. Then he went through hell to try and stop Thanos, came closer than anyone but blew it. Retreating in shame from what's left of his people makes sense. I can imagine he might start drinking and letting his hair grow unkempt. I don't have a problem with any of that.
My problem is with encouraging us to laugh at the fallen hero by putting him a fat suit and making him the butt of half the film's jokes.
My problem is with having this fat slob version of Thor time travel back to Old Asgard and still somehow be worthy of Mjolnir, which cheapens the lore of the character.
My problem is that rather than giving him an arc, they make him oblivious to how far he's fallen so that he's still being an alcoholic slob even after his friends bring him hope the snap can be undone and arrogantly claiming to be the 'strongest Avenger' in the third act.
Personally I've never found it funny when comedies put actors in fat suits but it's grossly out of place in all the serious scenes Thor's in and there was just no need to make one of the MCU's main characters - a god who's actually known for drinking mead by the barrel in both the comics and the myths without any ill effects - into a fat joke.
If the writers wanted to weaken Thor so that other characters could shine there were far better ways of doing that (injure him, have him unworthy of Stormbreaker, have his PTSD effect his ability to control lightning etc).
