I mean, was the pizza and video games that bad for his overall powers? Or was Thanos really that much stronger? Captain Marvel and Scarlet Witch both overpower Thanos during their little 1 on 1s. Why was Odinforce Thor no match for an Infinityless Thanos?
I don't think it's the pizza and video games so much as Thor has not been training or fighting in the intervening years, meanwhile he's going up against a Thanos younger than the first Thanos who defeated him in Infinity War. If you take a boxer and have him not train for years, then put him immediately back in the ring, you're probably not going to see his best effort in a match with someone roughly on par with him in his prime.
We saw in Ragnarok that even a powered up Thor still isn't powerful to defeat some beings like his sister Hela, so he's definitely not unbeatable.
Re: Scarlet Witch & Captain Marvel
Scarlet Witch fights better at a distance and Thanos is a close fighter, so obviously she had the advantage in that fight. He couldn't attack her and she could lock him down and attack at will.
As for Captain Marvel, we saw in Infinity War that the Infinity Gauntlet does not make you invulnerable to surprise attacks, and it could be simply that Captain Marvel blitzed Thanos and she was an unknown factor for him. Sort of like how Thor ambushed Thanos even with the gauntlet and nearly killed him.
Maybe 5 years of boozing and pizza resulted in more than 100 or so pounds of belly fat. And maybe 5 years of sitting around caused the loss of some muscle fiber, although I would argue that he is effectively in his 20's when compared to our life time (1500y ÷ 5000y = .3 times 100y for us makes him 25-30y equivalent). 5 years for him is about 1 month equivalent. No 25 year old loses any significant conditioning in 1 month.
His problem is mental:
- He lost big time to Thanos once.
- He failed to stop him the 2nd time.
- He killed him when he (Thanos) was a shadow of his former self.
- He spent 5y considering himself a failure to the entire universe.
- He is given to panic attacks when things get tough.
- His mother is the only one to talk sense into him at this point and she is dead.
All this makes him much less effective as a warrior than he was 5y before. This fight 5y before and Thor would have taken Thanos apart. He only lost the original battle because Thanos had the Power Stone.