Is "meh" an opinion or nothing? If it were no reaction at all, it wouldn't be audible, so it is a reaction of indifference. "I have noticed this and while it affected me enough to respond, it didn't make any other impact on me." So why respond that way actively when no response at all expresses your indifference even better? The only reason must be to make sure someone else knows that what they've said is a pointless waste of your time. That seems kind of harsh to me.
Mokalus of Borg
PS - Someone is sure to say "meh is just meh, you know?"
PPS - To them I say "meh".