is copy keywords from other people right thing or not ?
Is that a copyright ?

Of course keywords are not copyrighted, they are just common words not intellectual property.
You should definitely observe and study how people keyword their successful assets, that's how you learn, especially if you are not native English speaker.
Also there are many keywording tools made just for that.
All that said, I don't think it'll do you any good in sales if you simply right out copy keywords from other images without adding your unique ones. 

Guilty, Your Honor. I want to upload a photo of a "sliced tomato" for a first time, why would I reinvent a wheel? I grab keywords from top images, sort them, takes keywords that I don't like and could add mine keywords. Keep in mind that English is not my first language. I may just don't know the words that describe "sliced tomato" in details.

I look at other people's keywords, and sometimes I find a good one I missed, but usually I am just overwhelmed by the amount of poor keywording or outright spam. I guess that is what the sites should expect for pennies, but it is still sad.

As a beginner, it is certainly understandable to look at the keywords of others or to use keyword tools. I have done the same.
In the long run, however, this is not a good strategy. Especially with banal motifs, everyone ends up using the same keywords.

Many of my bestsellers are hackneyed motifs that sell exclusively on the basis of keyword combinations, mostly of a symbolic nature. Of course, I won't reveal which ones they are 

But to stay with the above example, tomatoes are symbolic for example of (spiritual) fertility, I would try that. And with orange peel you can also think of cellulite 

But I'm probably the only one here who enjoys keywording.