"Meh", "Ugh", and other actual emotions don't seem to fit my idea of what the other person wants to hear. Those invite conversation, or almost hint at asking for a conversation. I don't think that's what the other person usually wants.
I've taken to mechanically translating "how are you?" into an emotionless "hi".
I agree though, it's all very weird. Americans don't seem to just say "hi" very often.
In California, where I grew up, it's expected to have a small response:
A: "Hey, how are you?"
B: "Good, and you?"
A: "Good, thanks."
But in the Northeast, you'll stress yourself out trying to actually respond. People will throw it out as you're walking by each other, and if you try to squeeze in a "Good, and you?" they'll be taken aback because they're not really expecting to say anything further. And they're not really waiting to hear any actual response like "Meh" or "Woo".
In Boston and New York, I literally translate "How are you" to "Howdy" in my head. That is, "Howdy" was derived from "How do you do?" but obviously you don't respond "Good" to "Howdy". It's just a synonym for "Hi."
A: "Hi, how are you?"
B: "Hi."
I have found that the literal interpretation of "How are you?" can be countered with a rough summary of your internal emotional/mental machinery by way of an emphatic grunt or groan. "Meh.", "Woo!", "Ugh." are a few good places to start. Get creative, do some homework.
Remember this is a very basic, mammalian level trait. This is after lizard brain stuff, but before primate brain stuff. We don't need a lot of words for this particular relationship transaction. Have fun with it, but don't stress it. The person asking the question is certainly not stressing it.