Not currently
Its domain (www.diebergkomter.nl) is currently for sale, a strong indication the initiative has run out of steam. But that doesn't mean it was never considered.
Not officially
It has never been an official proposal or even idea by the Dutch government. Creating a new province, Flevoland, out of an inland sea, was. Building the Delta works to protect the provinces of Zeeland and Zuid-Holland from the sea, was too.
Now it's hard to prove an absence, but in all news articles I could dig up, no governmental involvement was mentioned.
But feasibly
The original proposal was made in jest, in a July 2011 op-ed on the Dutch news site Nu.nl. It was called "Berg!", which translates to "Mountain!" and is still accessible on the site.
It was met with enthusiasm by more people than Thijs had anticipated. Several sporting federations were interested. Technical universities were too. Feasibility studies were done. The proposal was settled on a hollow mountain; a solid mountain would be much too heavy, depressing the land in a 50 km. radius. A hollow mountain would also allow for the inside to be used for agriculture or energy storage. The most likely location was decided as off the coast of Noord-Holland.
Not anymore
The latest news article I could find, was from December 2012. It read that Thijs Zonneveld had let go of the idea as unattainable, mostly because of its costs — an estimated €4-7 000 000 000 000.
Conclusion
The best description of the seriousness of the project was perhaps given by Zonneveld himself, according to Reuters:
"It was not serious but the next day there was such a serious response from people who had actually been thinking about it and calculating stuff that it made me realise I was not the only one who'd had that idea," Zonneveld said.
( [...)]
"People may think this is a publicity stunt but this is not true. Publicity is the means because if people don't know about it, it cannot be done. The goal however is to build this mountain," Zonneveld said.