First, Portia's speech is not saying that the quality of mercy is uncontrollable or that mercy is not what it seems to be. Mercy is what it seems, a grace of God.

In this speech, Portia is arguing that mercy is more important than justice. She says that mercy is not strained, meaning it is not under the pressure of being over-used: people are not generally merciful. She then likens mercy to a refreshing rain. Like a gentle rain, it is a grace: not something that a person deserves, but a beautiful gift that a person receives from heaven whether they deserve it or not.

Portia does tie this concept of mercy to kingship, noting that mercy is most beautiful when shown by a person with a great deal of power, like a monarch. A monarch can exact punishment and justice, so when he shows mercy, that reveals his strength and goodness of heart. A merciful monarch is a good monarch because such a monarch is exhibiting a Godlike trait:

It is an attribute to God himself.
And earthly power doth then show likest God’s
When mercy seasons justice.