I had to search for it but
found it here. One interesting point in the poll, of those that say they cheated: "19% of Republicans, 16% of Democrats and 17% of unaffiliated voters who cast 2020 mail-in ballots say they signed a ballot or ballot envelope on behalf of a friend or family member. On the question of voting in a state where they were no longer a permanent resident. more Republican mail-in voters (24%) than Democrats (17%) or unaffiliated voters (11%) admitted doing so." So, if we are to believe this poll, then Biden still would have allegedly won since more Republicans cheated.
Of course, the flip side of this is, just because people allegedly tried to do this, there is no indication that these votes were accepted as valid votes, or if they were rejected due to signature not matching or other issues. For example, in Georgia (one the states where "fraud" has been claimed, such as people voting from out of state), the Georgia Bureau of Investigation did an audit of a percentage of ballots. In their investigation, the examined 15,000 ballot envelopes to ensure the signatures matched and, while they found a couple that they felt did not match the signature on file, they also contacted the voter listed on the envelope who verified they had signed the envelope. This would make it appear, at least in Georgia, that any fraudulent ballots received were unlikely to have been counted.