Mercedes is a common Spanish first name meaning "mercies" and is the short version of one of the titles of the Virgin Mary. But then, a lot of Spanish names for girls are titles of the Virgin Mary.
Conversely, Porsche is named for Ferdinand Porsche and not the female name.
The female name is normally spelled "Portia", from the main character in Shakespeare's The Merchant of Venice. Haven't you ever seen A Fish Called Wanda??