No it could have ended up worse for them, it might be the state of mexico instead of the nation of mexico today (Which since they all want to come here anyway probably would have been a good thing). The United States was being generous in giving them that, they could have just taken it.
Then, Arguably, the USA did "take" it (paying them $15,000,000 for approximately 1,000,000 square miles).
They act like if they kept that territory mexico would be a world power and economic dynamo today. No, now a bigger swath of land would be a shit hole.
Mexico played with fire and got burned, plain and simple. Santa Ana stupidly recognized a sovereign Texas, and when Texas offered to be annexed the US jumped on it. Not the fault of the United States in any way shape or form. Mexico then cried foul and attacked US soldiers in Texas and brought the wrath, in times when the WORLD was far less tolerant hippyish and PC. Britain and France (who also recognized the independence of Texas mind you) urged Mexico to quit fucking with the United States, they knew this would happen.
To the winner go the spoils, thats the way the cookie crumbles. My point is mexico instigated the war, they dont get a do over because the outcome sucked for them.
The winner also gets to write history as they see fit. In Mexico they refer to the Mexican American war as La invasión estadounidense de México (The American Invasion of Mexico). Some of the accounts I've read concerning what happened vary widely from what I'm reading here, others don't. I'm sure that many Southerners view the "history" of the US Civil War quite differently than the "official" version of it.
Also its rather dubious to surmise that the areas of california, nevada, newmexico, arizona et al were actually "governed" by mexico.