Can I sign my mothers I-90 form if she's physically unable to sign?

Are you a legal guardian?

According to the USCIS, Applicants and petitioners must sign their respective applications and petitions. Legal guardians may sign for individuals who are incompetent to sign. In such cases, the filing must include evidence of parentage or legal guardianship. The signature must be an original signature. CITATION: http://www.uscis.gov/forms/signature-requirements-uscis-forms