I don't deny that Thor was still Asgard's greatest warrior and a formidable opponent by the time of Ragnarok, he did after all give Kratos not one but two tough fights, he even managed to beat Kratos to the brink of death before reviving him. He was most definitely a worthy opponent for Kratos.
I still however believe that Thor was well past his prime as a warrior and his glory days were over as conveyed by his rotund physique and heavy drinking; he was not always fat and in fact did have a muscular build during his prime years as suggested by the statue in the first game. I felt that Thor longed for the days when there were enemies to fight, particularly enemies powerful enough to force him into exerting himself and allow him to feel jubilant in defeating them; that is why he really wanted to fight the Ghost of Sparta who Kratos had kept buried. By the time the game took place, almost nothing was a threat to Asgard besides Kratos, Thor could hardly have a fight he really wanted and all the fodder throughout the nine realms gave him zero thrills; he felt his warrior identity waning as he faced no challenges and had began drinking more and more heavily to drown out his sorrow much to the dismay of his wife and daughter.
I believe the Thor in Ragnarok was a far cry from the one who wiped out the giants and shook the nine realms, I believe Kratos only beat an out-of-prime Thor. I'm hoping we get to see prime Thor in a Tyr prequel animated series or comic.