Hi there,
There is no really a bad time to visit the Netherlands and it depends more on what you like doing. A popular time to enjoy a vacation to the Netherlands is summer and spring when you can enjoy biking through the country, as much of the area is quite friendly to bikers and trails are well marked.
During the summer, the weather is warm and activities such as an Amsterdam canal tour or Port of Rotterdam Harbor tour are easily arranged and enjoyed.
Having said this, travel to the Netherlands in the summer is expensive as
hotel and airfare go up due to the peak season.
In the other hand, winter is also a great time to be in the Netherlands. Some travelers head north by taking a
cruise. There are a number of cruise companies offering cruises that end in The Netherlands, but stop even further north in the winter to enjoy the Northern Lights.
Hope this helps
Vacationbuddy