"Legal Alien" by Pat Mora is about being both Mexican and American; she points out that, though Mexican-Americans have the traits of both people, they're fully accepted by neither. The theme of the poem is this dual identity, and the tone is at once frustrated and direct.
The theme of "Legal Alien" is the dual identity of Mexican Americans. The speaker talks about the ability to fit in with both English and Spanish speakers yet not exist as fully accepted by either because of the other cultural identification. Both groups believe that the speaker is different and, thus, can be fully accepted by neither. This leads to alienation and judgement.
The frustration is evident when the speaker talks about how Americans view her as exotic, different, and inferior. Mexicans view her as alien and different from them, even if she is capable of speaking like them. She points out that she has to mask her discomfort, knowing that people are already judging her before they know her. Mora doesn't, however, offer a solution. Despite frustration, it's clear that it's a larger issue without an easy answer.
The speaker's tone is also direct. She doesn't pull punches when she says that Americans may see her as inferior. She says clearly that Mexican people convey with her eyes that "You may speak Spanish / but you're not like me." Her discomfort is something that she's open with and not willing to deny to her reader.