I don't think there is a right answer, as different players and groups have different assumptions about what is "good." Some feel that a character that doesn't have at least one 18 and average of 14 or higher sucks; others feel that a True Roleplayer wears bad ability scores like a badge of honor, and exceptional ones with shame.
Problems generally arise in two ways: One, when there is disagreement in-group about the basic assumptions; and two, when a player is significantly above or below the rest of the group and someone is unhappy about it (I emphasize that and).
I think it really comes down to what the basic assumptions of the campaign are, as the DM and players want it to be - not as the rules say, or as whatever one's perception of the larger D&D community is. Are PCs meant to be "average people" or overall more talented and capable than average? And if so, to what degree? As with most elements of a campaign, this starts with the DM's vision of the campaign, and then adjusts accordingly with player in-put.
As with 99% of such things, there is no right answer, just campaign and group-specific agreements.
Speaking for myself and as a DM or player, I tend to prefer when a group has some variability in scores - because random generation is fun - but that the range isn't too wide, maybe something like average scores ranging within 2 points of each other (e.g. all players with an average of 10-12 or 12-14, etc). My personal preference is 12-14 for most campaigns. Or, as a baseline, arrays like this before any adjustments:
16, 14, 14, 12, 10, 8
18, 14, 12, 12, 10, 8
16, 16, 12, 12, 10, 8
16, 14, 12, 12, 10, 10
All total up to 74, or an average of 12.33. Or I might simply say, "take X points and distribute them as you like." And yes, that means a PC could have three 18s and three below average scores if they want. Or "roll sets of 4d6 until you come up with your first score that averages 12-14 or simply 12 or higher."
