It's not villain decay if the villain in question wasn't superior to the force they lost to in the first place.
If the Borg lost in normal combat to the Feds repeatedly, then that's villain decay (and it didn't happen). If the Borg lose to super-powerful aliens from another Dimension who were far beyond them because of inherent weaknesses the Borg have (but others do not have) and said aliens lose to others who do not have the Borgs' weaknesses, that's not villain decay.
If the Borg losing to ANYONE is enough to ruin their image, then the Borg were crappy villains/characters to begin with that it takes SO LITTLE to ruin them.
You don't see anyone complaining whenever Dominion soldiers got killed.