I can't speak for whomever should. But, making something for your own personal use shouldn't ever need permission. Only when you sell it should it matter.
If for some reason it does matter, you still have three options: buy a NetBSD sticker and stick it on; buy the sticker but throw it away and consider the purchase a licence to use the image that was on the sticker, once; or, buy a NetBSD logo mug from the official source and attach the other logos to it.
