That is true... I don't know of any examples of magisters targeting the poor of Tevinter... I thought that was the reason they had slaves in the first place. So they wouldn't need to attack their own citizens?
Soporati are not the poor class either, Most are Merchants or in the army they can own land and have full rights apart from how high they can go in the Imperial Chantry and they cannot have a direct say in the government.
Not only that but if a mage is born into a Soporati family the entire family gains entry to the Laeten class where they have more rights.
the poor would be Liberati class it's made up of Free'd slave's that are not citizens (and as such cannot own land)
Considering the theres likely many Laeten's in the government, Who have Mundanes as family members and those Mundanes belong to the exact same cast as Magi Laeten's with the same rights (minus being in the government) I find it highly unlikely that they would allow Attacks on freemen for simply being Mundanes.