What influence and/or impact did one or both of Martin Luther King, Jr. and Malcolm X, have on Mendez v. Westminster contributing to civil rights?
Neither of these men had anything directly to do with this case.
This case was decided in 1947, before either Martin Luther King, Jr. or Malcolm X was a widely-known figure in US society. In addition, this was a case about the segregation of Mexican-Americans, which is not an issue that either of these men was closely connected to. We must also note that Mendez did not even directly challenge the Plessydecision that legalized the type of segregation that King would fight in the 1950s and '60s. Instead, Mendezsimply ruled that California segregation laws allowed the segregation of Asians but not Hispanics.
One can argue that King continued the work of Mendez by fighting against segregation. However, the connection is somewhat tenuous because the Mendez decision did not say anything about the general legality of segregation.