There were fraudulent ballots cast for both candidates [1]. I'm not accusing anyone of widespread collusion; I'm just saying that it happens.
Either way, I think "is not a crime" is a low bar; "is morally correct and aligns with democratic principles" seems more correct in this case. Our system is founded on the principle of "one person, one vote". Anything that isn't in accordance with that (e.g. mailing a ballot to a dead person), even if it isn't illegal, seems like something our society should minimize or have fail-safes to prevent.
[1]: https://apnews.com/article/voter-fraud-election-2020-joe-bid...
https://apnews.com/article/election-2020-government-and-poli...