Sanctions targeted Zimbwaweans and affected them. Whether they intended to target Mugabe and his allies..is irrelevant considering...as you said..they ended up sending 3m folks out as refugees..while Mugabe has won 3 times since then and his wife is set to win.
Targeted sanctions is moronic diplomacy. The reality is any sanctions ended up affecting the poorest.
The US embassy has laid out an objective case on the myths of the sanctions matter, with specifics. If you have an objective response, with specifics, then let's have it. "Sanctions affecting the poorest" is the standard but vague NGO-speak.
On a matter of logic: one must not assume that because 3 million+ people left during a period of sanctions, it necessarily follows that their leaving is related to do with sanctions. That is because:
(a) People do manage to ruin economies even without sanctions and consequently their people leave.
(b) Following from (a), one would have to start with an argument that the sanctions, rather than economic bungling, are what has turned Zimbabwe into an economic basket case.
(c) In (b), one should include a huge drop in agricultural output, hundreds of millions spent a military adventure in the DRC, etc.