Is it so hard to imagine that writers write primarily for dramatic effect, not power level consistency? Thor did well against Thanos because it served the dramatic moment to do so.
Again bringing up one of my favorite characters...Mark Schultz wrote a story in which Darkseid decimates Superman in 3 panels...and then later wrote a story in which Superman beats Darkseid so thoroughly that he has to beg for quarter.
Fabian Nicieza writes a story in which Apocalypse utterly destroys all foes in his path, and then later writes a story in which he's killed dead by Cable with a single shot to the head.
It happens. And, by itself, means nothing.
Gruenwald had a fued with Starlin, because "Infinity Gauntlet" usurped "Cosmos in Collision" (the superior story, in my mind) as the summer cosmic crossover event. That's why Starlin wrote that scene in which Quasar gets his hands blown off, where he's little more than a joke.
DC/Marvel was considered canon at the time of it's publication by Marvel editors. They said as much in the letters pages, and Access appeared a couple of times in Marvel comics as well as DC. Silver Surfer/GL, which lead to All Access, was referenced in a DC comic as well.
And yeah, the Kazar incident is wonky. But it also demonstrates that writers are capable of writing whatever, and that having varying power showings means nothing in regards to intent. Dan Slott had it stated without a doubt that Squirrel Girl defeated Thanos to, tongue and cheek though it was. And he later had an exact duplicate of Thanos, identical in every way, get beat up by She Hulk.