Loki's character got acclaim because he had an arc, a good story for his character. Joker and Black Widow were more flamboyant than the title characters of their last film appearances, but the main character had a better story, a full featured arc, and was thus at least as interesting, even though they were very straightforward characters. Thor is always defying his father and always seeing Loki as redeemable. There's no growth or progress or anything interesting for him there. Thor was absolutely less interesting, because he didn't have a good arc. He did stuff Thor always does, or something random with no buildup.
Loki didn't just steal scenes as usual, he stole the whole show. Loki on the throne is the resolution of the film. Thor reuniting with Jane is just an after credits scene. This makes sense because Loki had a solid arc in the film of growth that was new for the character and constant throughout the film. The fact that Loki is the only great thing about the film says everything that needs to be said about Thor:TDW.
No, Loki's character got acclaim because he's an inherently more interesting character. If you're defining a character arc in terms of character growth, Thor got an arc in every way Loki did. Loki was in Asgard-jail, grieved his mom, made a pact of convenience with his brother, faked a heroic death, pulled a few signature tricks, ended up on the throne. Thor started off keeping the peace in the 9 realms, visited his in-danger "girlfriend", whisked her away to Asgard, made a pact of convenience with his brother, defied Odin, saved the universe (unfortunately, with the help of Jane's intern and Jane's intern's intern

), rejected Asgard.
You're essentially reacting to the fact that you found Loki to be the more interesting character. So did most people did. And that's absolutely fine since he's the one who's always scheming, has the acerbic wit and portrayed by a fantastic actor. Thor on the other hand is the more traditional, straight-laced, headstrong hero played by a good actor. To summarize, you liked the schemes Loki pulled, you liked the wickedly witty dialogue and the performance. But "arc" is not shorthand for those things.
This is how film schools often teach their students to bifurcate a film's component aspects. The physical journey of a character is driven by story. The intellectual journey, by the plot. The emotional journey of a character is what goes into the arc. Personally, I think all if that is hokum and don't judge a movie in a paint-by-numbers fashion that boils everything down to a formula. However, since you place an inordinate amount of importance on "arc", you cannot, with a straight face, tell me that Loki's emotional journey over the course of the film was greater than Thor's.
Look, as I've stated before, T:TDW is a flawed film that is more intent hitting certain predefined marks out of workmanlike duty rather than investing in giving the audience a story that ought to be full of magic (magic as in film magic, not Asgardian science/magic) and wonder. Which, as an aside, I thought GOTG absolutely pulled off.
However, it really seems like you are calling for Thor to behave in ways that would go against his character. What you call the lack of an arc, in this case, would merely be consistent characterization.