Female changeling stupid??

[Deckerd]

The Nazis were in no way exceptional when it comes to a superior force exterminating the non-combatants of an overpowered region. It goes back to prehistory. Since ST is basically human stories with the baddies dressed up as aliens (because humans have put all that unpleasantness behind them), it makes sense to have one who thinks nothing of genocide. It's a familiar concept after all.

 

[Loskene]

Lakarian City, 2 Million Dead.

Guess she figured holding a gun to Carassia's head would be enough to keep them fighting on their side until the end of the war.

Total Dead on Cardassia following the battle 850 Million +

Quite an effective gun at that, the Cardassian's calling her bluff costs them 100-150 Holocausts worth of retribution. Let's not forget the most important thing, the Female Changeling didn't really care if she conquered the Quadrant or ruled it or even lived, she just wanted to destroy as much as she could with the very small portion of the Dominion she brought with her, most of her ships and soldiers were constructed in the Alpha Quadrant...

She succeeded in that almost entirely. If it wasn't for Section 31 forcing her hand the losses would have been much higher, the Jem'hader would have kept on fighting right to the end.

 

[Dal Rassak]

She's not stupid, she just does not understand how "solids" think.
Also the Founders have a massive superiority complex. All other lifeforms to them are inferior. The Cardassians were just pawns to be used from the start; it just wasn't obvious at first because Dukat could hold his end up with Weyoun. When Damar rebelled that was it. A pawn that refuses to go where you place him on the board is no longer useful.
As soon as the Cardassians refused to be gaming pieces, their usefulness to the Founders was at an end and annihilation the only answer. From their point of view the Founders had to "set an example", otherwise every other world they'd overrun might get ideas of rebellion into their heads...

 

[Jono]

The idea of levelling Larkarian City was Weyoun's idea. He was more or less running the Dominion for most of the war. He even worked closely with the Cardassians, but as we saw towards the end of the war he didn't understand them all that much. His actions created the initial military rebellion to begin with. So the failure is more on his head than the generally disinterested Founder.