I am looking at the I-130 form and the corresponding instructions here: https://www.uscis.gov/sites/default/files/files/form/i-130instr.pdf. While it is obvious that the petitioner has to sign the form, I could not find any mention of whether the beneficiary has to sign the form. Is the beneficiary's signature not needed at all?
No, the beneficiary doesn't have to sign the form. The petition is only filed by the petitioner and the beneficiary is not involved. Only at a later stage when a visa number is becoming available and the beneficiary is going through consular processing to get an immigrant visa will the beneficiary get involved.