AJT,
Wilde simply visited America as a tourist. travel broadens the mind and all that. This was just a throwaway comment. Niagara falls were (are?) a big honeymoon spot and this remark is one of many Wilde made slagging off marriage. He is also saying that the Falls are a great disappointment and that is true.
Like Dr Johnson said about the Giant's Causeway;"worth seeing but not worth making a special trip to do so. I've seen the falls and the simple fact is that too much water is exracted for hydo-electricity plants. What should be an awesome spectacle isn't.