Not at all plausible unless you have a very early, very minor TL that keeps the same general trend but either weakens the US or strengthens Mexico. The Mexican-American War is often called the "most unfair war in history." Remember that Mexico's economy was in shambles, their military had little to no motivation, and the Americans were on the exact opposite end of the spectrum.
If you have a very early POD, say in the 1820s, it could work. Either make Mexico much stronger, or have some sort of political shift in the US. I think that would be the more interesting one, have the political shift cause the Americans to completely butcher the war, resulting in a Vietnam situation in Mexico, to a point where the war just drags on, becoming more and more unpopular until finally, Abraham Lincoln is elected on his famous OTL anti-war platform, and stop the war so the nation can recover.
That'd be interesting: an earlier President Abe Lincoln with a weakened USA and a different slave-free situation.