Netherlands is showing great improvement in last matches they have given a tough time to Zimbabwe in recent series.
Zimbabwe on other hand has done good they have come in international cricket after many years and from that aspect their performance is good.
Rather than any improvement from Netherlands, for me it was more like deterioration of standards from Zimbabwe. Despite being a full member of the ICC, Zimbabwe can't compete against other test nations in a meaningful way. And even while competing against associate nations such as Scotland and Netherlands, they are really struggling. And another fact is that they are still dependent on players such as Sean Williams, Craig Ervine, Gary Ballance and Ryan Burl, who are mostly based outside that country. Local players are not very consistent (Blessing Muzarabani, Brad Evans.etc).