But the thing is, I don't think it's just unhelpful tutting (well, maybe some of it is, but not all).
The thing about labelling it "naughtiness" is it sets up the wrong solution.
Naughtiness, in my book, is when a child knows they are doing something wrong and does it anyway. The solution is to reinforce that what they are doing is unacceptable by whatever means floats your boat - time out, telling off, whatever.
The problem is, an 8 month old doesn't know what they are doing is wrong, and won't understand the concept of "wrong" if you tell them. By telling them off and saying "no" and "ah ah" you are just giving them a fun reaction and reinforcing that this is an exciting pass-time.
ultimately, if you don't want your child to touch the radiator, whack the telly, whatever, you can say "uh-oh" until you're blue in the face but it will not work until they are of an age to understand. As the OP pointed out, smiling and saying "ah ah" has had the opposite outcome to the one she wanted.
By pointing out the fact that the child is not being naughty, just curious, people are trying to help the OP arrive at a solution that will work. I don't think this is sanctimonious or preachy, it's being practical.