I was wondering if anyone knows if John Lennon became a U.S. citizen before his death in 1980. I read that when Nixon was president, he tried to keep him out of the U.S. because of an old marijuana possession charge, but after he left office, Lennon was granted a residence permit.

No, Lennon never became a US citizen - he would have been eligible to become a citizen in 1981, but didn't live long enough. He resided in the US from 1971 until his death in 1980 (largely to avoid UK taxes), and became a permanent resident in 1976. In the early 70's Nixon did try to have him deported, unsuccessfully, based on Lennon's 1968 misdemeanor charge for cannabis possession in London.