CA’s electoral block did indeed go to the GOP Presidential nominee from ‘68 to ‘88.
But the share of the Latino vote going to the GOP nominee did not appreciably change after 1992. That is not consistent with the hypothesis that proposition 187 is responsible for the GOP's electoral woes in California.
Here's another datapoint inconsistent with the prop 187 conventional wisdom. Pete Wilson, who strongly supported 187 when campaigning for governor in 1994, got a larger share of the Latino vote than Dan Lungren in 1998, who opposed it.