Is Steppenwolf stronger than Thanos?
This is going to require some context, but it'll work.
Do you remember when Superman showed up at the end of Justice League and laid the beat down on Steppenwolf? Superman wasn't even really trying there. One hit against Steppenwolf, which only launched the man hard enough to break a chunk of wall behind him, was practically enough to end the fight. Sure, Superman hit him more after this, but he didn't really need to. Steppenwolf basically lost after that punch.
Now let's look at Thor. Thor is so strong that he could hit the Hulk hard enough that he circumnavigated half an arena that looked to be about the size of a football field, destroying steel structures all the way through.
Infinity War literally opens with Thanos casually defeating Thor, handling him like a child. He didn't even notice Thor's attempts to beat him.
Thanos would hardly be staggered by Steppenwolf's blows. One hit like Superman had there and this fight would be over.