It's the old 'if someone bears thor they must be powerful dude' line as opposed to 'wow Superman beat darkseid, he must be a wuss now'
But if Thanos lost to Thor without extenuating circumstances, it would be a downgrade (at least from post Death powerup Thanos)....though he seems to be getting downgraded a bit these days anyway.
Its not that Darkseid is a wimp if beaten by Superman, its that there has been enough precedent on a lot of these showings to say that Darkseid is not substantially above Superman or Orion, and by extension, Captain Marvel etc. And I seem to recall some Jonn Jonzz love vs. Darkseid as well, but I can't for the life of me remember where.
If you don't believe those guys are all substantially above Marvel top tiers, then that makes Darkseid very beatable. He's not a wimp...it takes someone at the top end of the hero spectrum to beat him, but its questionable who is the underdog in such a matchup.