I was told that "мне хорошо" would sound very strange as a response to "как дела", and indeed I've never seen or heard in such a context, but I don't completely understand why. I've seen it in contexts like this:
Мне хорошо рядом с тобой
What is it about the construction that makes it acceptable in this context, but not as a response to "как дела"?
This response is wrong both grammatically and semantically.
"Как дела?" literally means "How well are your deals?". So the answer should be about the deals. For example you can answer "Спасибо, мои дела идут хорошо." "Thanks, my deals are going well". When answering "Мне хорошо" you do not answer the asked question, because it refers to you rather than the deals. That's why the answer is grammatically incorrect.
It is also incorrect semantically because "Как дела?" refers to overall state of affairs of your life, while "Мне хорошо" means you feel pleasure just now. In fact your affairs can be bad but you still can feel pleasure currently.
So the proposed dialog looks like:
-How well are your affairs?
-I feel pleasure now.
Really there is no much meaning in it. It makes impression that you are a drug addict whose business is to feel pleasure.
"У меня всё хорошо" will be OK - I guess this is a phrase you are looking for actually.
"Мне хорошо" is actually close to "I feel good", not "I'm fine".
So imagine a dialog A: How are you? B: I'm feeling good.
A little bit odd, isn't it? )))
Just to add to everything said above: "Мне хорошо" would be a fitting answer for "Как тебе?" (I can imagine this dialog after changing some physical environment, say, opening/closing the window: -Как тебе? - Мне хорошо. - А мне холодно/жарко.)
