That is not surprising since that seems to be what the statute is intended for. Have you ever seen that charge applied to someone trying to influence the outcome of an election? Mueller seems to be arguing that efforts to influence voters, since they might effect the outcome of the election, are a fraud on the government. That seems like quite a stretch.
‘Conspiracy to defraud the US government’ is colorful lawyerspeak for some kind of tax violation or illicitly obtaining government money. ‘Defraud’ legally involves financial gain or loss.