Thor’s whole Endgame arc gets misunderstood by people focusing on the beer gut jokes. His body wasn’t actually the problem, Asgardians don’t get weaker from sitting on a couch for five years, if anything they age like fine wine.
What actually happened is the guy watched his brother die, lost his best friend Heimdall, failed to save half his people, then had one clean shot to kill Thanos and aimed for the chest instead of the head.
That’s not a strength issue, that’s a man who genuinely believes he already lost the fight that mattered most. His powers have always run on confidence, not muscle, so of course he looks weaker, he’s fighting like someone who’s already given up before the punch lands.