A lot of the motels in the Falls change owners almost yearly. You cannot keep track of which ones will be clean or not, from trip to trip. There are some clean ones, but generally the cheap ones are just that...cheap. Going for the lower end hotels is a better gamble, cost more but cleaner.
Best advice though is not to pre-book, show up and drive around. Business is quieter this year, especially at the motels. Most have vacancies, and if you're not prebooked they cannot bill your card, might even get a better rate than posted.