I would argue that mercantilism was beneficial to both the colonies and the mother country but that it was typically much more beneficial to the mother country. Let us look at the example of the British colonies in North America to illustrate this.
In a sense, mercantilism was good for the colonies because it gave them access to the British market. So long as the colonists were producing things that the British wanted, they had privileged access to the markets of the richest country in the world. They did not have to worry about competition from foreign countries.
However, mercantilism was better for the mother country. It got to set rules that prevented, for example, any colonial firms from making goods that competed with those made in England. It got the benefit of cheap raw materials from the colonies and sold them more expensive finished goods in return.
Thus, mercantilism was an unequal relationship that helped the mother country more than it helped the colonies.