To answer the main question:
Voting for Biden could be a mortal sin if one does not have a proportionate reason for materially cooperating with the grave evils of abortion, embryonic stem cell research, and the persecution of nuns, religious, etc.
Chiefly, abortion though. When one is considering abortion while voting, one must take into account the Church teaching. The Church teaches that abortion is the murder of an innocent human being. There are about 600,000 abortions in the United States every year. So abortion is a legal genocide on a large scale. “I don’t like Trump” or “Trump said mean things” or “I think Trump is a bad President” are not proportionate reasons for supporting a candidate who not only supports legalized abortion, but wants to subsidize it with government funds. If Biden had the same policy positions, but wanted abortion to be illegal, and instead wanted 600,000 black people to be able to be legally slaughtered every year and for the government to subsidize that choice, would your answer change? If it would, then it seems your vote goes against Catholic teaching, and your vote would be sinful, potentially mortal because of the gravity of abortion.