No, absolutely not! The surname of any individual, family, or creed does not necessarily dictate religion, race, nationality, or politics. You would still need to search the name in its full genealogical placement and determine from historical records (Church records, etc.) what the religious affiliation of the surname pertains to.
For example, in my genealogical line, the surname Ramirez (also spelled Ramires)in the United States (Texas) is normally accepted as Spanish or Mexican-American. Yet, within the same State and locality that are some with the same surname but they are Italian (Rammires). The majority of the Ramirez line is Catholic. Yet some are Baptist, Episcopalian, Methodists, Pentecostal, and Mormon. If you go back one full generation, you will find the majority of Ramirez relatives in Northern Mexico belonging to the Catholic Church (based on Church Baptismal records). But within the same locality there are some that are considered "Christian" implying that they are not Catholic, but Protestant. As you trace the Ramirez line through Spain and Portugal, you will find that the majority are Roman Catholic. Yet, within the same locality there are some that are Greek Orthodox and Protestants (primarily Methodists). Going back further in generations places us in Northern Africa (Morocco) and the Ramirez (now spelled Ramerez) are considered Sephardic Jews. If you study carefully the nature of these Sephardic Jews, you will find through autobiographies of a number of these individuals that they are not Jewish, but rather of the Moslem race. They were called Sephardic Jews as an insult to them because they chose not to follow the Roman Catholic Church. They chose to follow the Greek Orthodox Church. Yet, within the same locality there are indeed some that are Moslem, and have some linkage to Beduin tribes of the Sahara Desert. If you trace the line back further, you will find the line going into Algeria. Here you will find 3 major groups: those affiliated with the Roman Catholics; those affiliated with the Moslems; and those affiliated with the Druzas (of the Middle East). So the question that you posed, "Is the Ramirez name of Sephardic Jewish origin?", provides with only one answer: "Yes, if you trace the line far enough back so that you find a linkage with the Sephardic Jewish "traditions" (religion). However, you will also find Roman Catholic, Protestant, and Druza linkages which give a fuller picture of the Ramirez surname.
Remember, that Sephardic Jews came as a consequence of the Romans exiling the Jews into a number of European countries. Those that were exiled to the Iberian Peninsula were called Sephard (Hebrew word). The Jews in Spain and Portugal were called Sephardim (or Sephardi). By 1213 AD the Sephardim had been recognized in Aragon as "sephardic in nature." In 1492 AD there was an expulsion of the Sephardic Jews from Portugal and Spain. Those who promised to become Catholic were re-issued their civil rights but were now called Marranos ("pigs" in English). Later, "Clean Blood Laws" were enacted in Spain and Portugal to deny the Marranos (or New Christians) the same civil rights as the "Old Christians." With this historical perspective in mind, you can see that it is quite possible that many converted, and many did not and chose to live a secluded private life, practicing their Jewish beliefs in secret.
So what surnames are considered Sephardic in nature as found in Spain and/or Portugal. The following surnames demonstrate the cause that Ramirez was probably Sephardic in origin: Ramirez Cartagena; Ramirez de Portella; Ramirez; Ramires; Ramirez de Guzman; Ramirez Lopez; Ramirez de Montilla; and, Ramirez Montilla. There are a number of genealogical sources that prove these name to be Sephardic in origin. Whether they were truly Sephardic Jews or Catholic can only be proven by their autobiographies and the celebrations that they observed on an annual basis.
I hope this helps. It is more complex that just assuming a surname is Sephardic, Spanish, or Christian.