Whether the Mexican people as a whole have any claim to begin with is certainly worth debate, since the Natives were here before them and would certainly contest that claim. However, what is not in doubt is the fact that certain people and their heirs in the now-U.S. southwest were granted land by the former Spanish government a hoop-long time before there ever was a U.S., back when they were settling the area (or invading it, depending on your point of view); that those grants remained in effect under the later Mexican government; that the treaty which ended the Mexican-American war obliged the U.S. Government to honor those grants; that the U.S. was therefore bound under U.S. law to honor those grants; and that some of those grants were not subsequently honored. The heirs of the grant-recipients can rightly claim that the land was stolen from them because the U.S. government specifically agreed in treaty to honor their claims and then violated its own law by not honoring those claims. (Refer to Article-VI of the U.S. Constitution.)

Now, whether it is proper or not to honor that claim at this late date is an issue for the courts. There is an argument to be made that something like a statute of limitations should exist on righting ancient wrongs, or else we'd be in the position of trying to sort through claims of some Native tribes that they were displaced thousands of years ago by other Native tribes and should be given their ancient haunts back. However, the claim that the land was unlawfully taken from those Mexican and Mexican-American heirs who held proper title has unequivocal historical documentation to support it.