Why is Sodium acetate called a salt of weak acid and strong base, when Acetic acid behaves as a strong acid (i.e. it shows almost cent percent dissociation) in Sodium hydroxide solution ?
Your definition of strong acid is wrong.
Simplistically speaking, it is defined with respect to dissociation in water, not in NaOH.
Acetic acid only dissociates partially in water, so it is a weak acid. It doesn't matter whether it dissociates completely in NaOH or not. That's irrelevant.
You are right. In sodium hydroxide solution, you should not call sodium acetate a salt of strong base and weak acid. If so then the pH should be even more greater than what we expect. However, the pH should be less than expected, though not appreciable. It is because the ionization of NaOH will compete with the ionization of sodium acetate as well as with the hydrolysis of acetate ion.
However, defining of strong or weak acids or bases is done in water not in sodium hydroxide solution.