A 6/10 game can maybe get really good, it obviously depends on exactly what the issues are, if they can even be fixed or if will be some half-assed hack that just makes it 25% better. Games can have many small nuances, maybe it's total garbage AI, or garbage UI which just ruins an otherwise good experience.
You mean some games can have small blemishes that negatively impact the overall experience, and thus, small little mods can iron out those issues and lift it from a 6/10 to potentially an 8/10 etc.
The issue I see is that most people use Mods as a crutch to justify developer laziness, Mods are supposed to enhance/add-on to the original game, not replace it altogether.
at that point, you're playing a completely different game that its strengths and weaknesses cannot be attributed to the designers because you've modded the game so much it's nothing like the original product anymore.