There is, of course, no objectively correct answer for this. People can and do argue this from both sides.
On the one side, we can say Ford’s action was improper. Nixon had almost certainly committed crimes. His crimes might not have been that serious if they had been done by a lower-level person, but Nixon was the President. This makes his actions more serious because he should be more likely to uphold the law, not less likely. Letting him face charges for his crimes would have sent a message to the country that politicians, no matter how high up, are not above the law.
On the other side, we can say that Ford did the right thing for the country as a whole. From this point of view, prosecuting Nixon would have been needlessly divisive. It would have deteriorated into a partisan squabble of epic proportions. It would have kept Americans’ focus on the scandal and not let it fade into the past. From this point of view, Ford was correct to pardon Nixon and to, as he put it, end our “long national nightmare.”