Surtur did look quite weak, and he struggled to get up and approach Thor with a sword. Once it was time to fight, Surtur couldn't even fight back, and Thor ended up defeating him and taking his crown with him.

Why can Thor beat Surtur but not Hela?

So Thor is fighting Surtur at only a fraction of his true power, while Hela is fighting him after he's been reunited with his Eternal Flame, and is at his full power. Before Odin trapped him, he had been powerful enough to destroy all of Asgard by himself if he wanted, so once he's restored, Hela is no match for him.

Can Odin beat Surtur?

Odin is the only being known to have been able to defeat Surtur at full power, having depowered, wounded, and banished him thousands of years ago. ... Odin, however, was able to severely wounded him at full power, leaving him battered and hobbled for more than 5,000 years.