There isn’t a bad time to visit Hawaii — at least, not where the weather is concerned. Hawaii’s tropical climate means it experiences warm, sunny weather year-round. Although the archipelago does experience a rainy season, tropical storms are few and far between, ensuring fun in the sun from January to December.
That said, if by “bad” you mean “expensive,” the winter months (December through March) tend to be the priciest times to visit and generally the worst in terms of available deals.