If a political party (Party A) has a nominee that Party B does not like and Party A’s candidate wins by a relatively small electoral margin (318 to 220), can party B appoint sham electors to vote for their candidate? Here’s what I mean: using control of state legislatures to get its candidate into office. So, they would hypothetically have enough control to get them elected. As far as I know, the constitution of the US doesn’t explicitly prohibit such a move.
Even if this were to be done, Congress has to confirm the result of the electoral college vote before it becomes official.
Some detail in this other question: A small set of questions on objections to the Electoral College vote by members of Congress
So, while it's possible for state electors to be picked for undemocratic reasons, the assumption is that a member of Congress would raise issue with it and not count those votes.