It's the philosophical problem of acts vs. omissions. By failing to act (i.e., vote for Clinton), you share in the culpability of the bad outcome (A Trump victory).
There are some problems with this line of thought. Are we morally responsible for the death of children in faraway places because we don't give enough to charity? Suppose there's only one charity and it's really corrupt, but it still steers 10% of its money to stamping out malaria. Should one support it anyway?
I'm not going to tell you you're immoral for not voting for Clinton, or you have a duty to vote for her to prevent a horrible outcome. I think you should, but I understand if you can't bring yourself to do so.