"Dwelling in tents" implies that Jacob stayed home and attended to his cattle. But in what sense was he a "plain" man?
The Hebrew adjective is tam, which means complete, perfect. It is the adjective applied to Job, when the LORD calls him "a perfect and an upright man" (Job 1:8; 2:3). It is the adjective used in Psalms 37:37—"Mark the perfect man, and behold the upright." RSV reads "blameless" instead of "perfect" in these verses.
It seems clear that Genesis 25:27 is to be taken as a quite objective statement. The writer is not here concerned with the moral character of either of the twins; he is not apportioning praise or blame. He is simply stating the basic difference between their respective interests and ways of life.
In contrast with Esau's special craving and skill, Jacob was an 'ish tam, a complete man, in the sense that he took seriously life's ordinary duties.