A lot of people decorate their bikes here. Not usually to the extent of this one, which was in a competition in Assen, but nevertheless a lot of people do decorate them in odd ways.
This varies from painting them in odd colours (a way of "renewing" a tired old bike, and perhaps scaring off thieves), to adding plastic flowers. In fact, the flowers are the subject of "Het Blije Fietsenplan", a mock cycling plan based on making "happy bicycles" with flowers attached. The website hasn't been updated since last year, but if you've not seen it before, it might be worth a look, especially at the fotoalbum.