The fact is that any first person account in a story can be considered unreliable. This is because we are seeing the action through the eyes of one character. Any person, because of their own unique set of prejudices and world vision and outlook on life, is going to have a skewed perception of reality. Therefore, if we are given an account from the perspective of one person, it will be unreliable to some extent. What we have to decide as readers is how unreliable the account is or not.
If we look at the character of Jim Burden in this text, it is clear that he exhibits the class attitudes that we would expect him to display in this novel. What is also interesting is the way in which he alternates between being repelled and attracted to the figure of Antonia. On the one hand, he is intrigued by the force of her character and will-power, but on the other hand he finds himself repulsed by her outspoken way of speaking. This is one way that we can definitely identify Jim as an unreliable narrator because of his own prejudices and outlook on life.