No, there is not a bad time of year to visit the Oregon Coast, but there is a rainy season. Between November and January, many parts of the coast average over 10 inches of rain each month. Many people come to the coast during this time to experience the awesome storms that can whip the seas into a frenzy and showcase the dramatic forces that shape the landscape. It can be wet and windy, but many are attracted to this quieter season at the coast. Hotels offer great off season rates too.
July and August are the most dependable months for dry weather and sunny skies. It is also the busiest time of year. My favorite time of year is September and early October. There are typically smaller crowds and the weather can still be nice, though slightly cooler than summer. Fall can offer scenes of fog, mist and cloud formations that can be stunning.
Whatever time of year you come, you will want to be prepared for cool temperatures and the possibility of rain. If you are prepared, you will be able to enjoy the magic of the coast in any season you visit.