system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU] Do the Guardians know how powerful Thor is?
gpt
They have a vague impression, but not a clear picture.

When they met him, he had been floating in space for an indeterminate amount of time without suffering harm. His response to this ordeal was to ask for a cup of soup and a space shuttle so he could go grab a weapon powerful enough to kill Thanos. Assuming they believed that he was actually capable of that (and Rocket at least seemed to), that alone puts Thor near to top level of anyone they've ever faced. Rocket has also seen Thor unleash hell with Stormbreaker, so Rocket knows that Thor is at least an army killer. 

I don't think he actually saw Thor and Stormbreaker overpower the Infinity Gauntlet, though, which would raise Thor from "very impressive" to "pants-shittingly impressive." And Peter and the others weren't un-snapped until after Steve, Tony, and Thor did most of their fighting against Thanos is *Endgame*. So for most of the guardians, Thor's biggest feats were off-screen.

Also, Thor is the kind of guy people underestimate. He doesn't take many things seriously, and it's easy to think of him a joke, not a warrior-king. This is especially true after his five-year Lebowski impersonation. Very few people have seen him rage hard enough to blacken the daytime skies, and even fewer have lived to tell about it.