I think it is Mr. M. who kindly advised me that an adjective have quotation marks on it if the usage is unusual.

In respect to this unusual usage, would you say it is in terms of the word's meaning? Or could it also be in terms of its grammatical correctness?

John is a 'superb' actor. -- I think in this case, a person could easily have written "John is a superb actor" without quotation marks but has done so because mainly to magnify the effect the word 'superb', in other words, makes it stand out. Is this a correct reason for the use of this mark?

We need to do feedback evalution based on the responses we got.

This is what troubles me: You have this word 'feedback' and normally you would not see this word with the word 'evaluation' but in this case, a person obviously used it to say what he wants to say but to me, the grammatical usage (if I can call it that) is awkward. I usually perceive the word 'feedback' as being a functional noun in its own right. Since I feel uneasy about its grammatical usage, could I be able to put quotations marks around it? Is that a correct reason for the use of this mark?

We need to do 'feedback' evaluation based on the responses we got.

Thank you.
In the case of your example with superb, I think you are using what are sometimes called scare quotes. They can be paraphrased by preceding the word with so-called. That is, they show that the author does not really believe what he's quoting. He is implying that less informed people might believe it. Or, in less extreme examples, the author is merely drawing attention to the fact that many people use the word, and perhaps incorrectly or in an exaggerated way.

John is a superb actor says that John's acting is excellent.
John is a 'superb' actor says that he is a so-called superb actor, that is, he is regarded as superb by some, but they are probably wrong in the opinion of the author. This is totally a written effect; there is no easy way to phrase it with your voice if you actually say it. (The sentence is actually not the best example of this kind of usage.)

I don't see any reason to use scare quotes on feedback in your other example.
(I have no idea if this is the use of quotation marks referred to earlier by Mr. M. The two of us may be referring to two completely separate stylistic phenomena.)
Using markers around adjectives should be done very carefully. In addition to what Jim said, I'd say that:
John is a 'superb' actor
may be written in order to ridicule the assessment made, or the manner of it being said, as in:
She said John is such a 'supeeerb' actor
If you don't want to introduce doubts, I would not use markers.
In your 2nd sentence, using "feedback" evaluation would only make others try to figure out what do you mean by those quotation marks, i.e. perhaps that you think that feedback isn't the right adjective?