I always thought the main goal of sanctions was to deprive the sanctioned governments of income which in turns makes it harder for them to cause problems externally and hold power internally.

And by "hold power internally" I don't mean population uprising, I mean to keep the factions within the government (especially the military) united under the leadership by buying them out.

I believe increased population unhappiness is more of a side-effect that can be both beneficial (if it incites anti-government sentiment) or detrimental (if it incites nationalism) to the country imposing sanctions.