Based on the suppliers Nintendo uses (the vast majority is public information), they are already likely pretty high up in the list (it's pretty easy to figure out). The difference is that they'd rather not engage the NGO in question, likely because it opens the door to 1000 other NGOs trying to demand Nintendo comply with their own ranking systems or face a boycott of some sort. Hence the NGO is giving them a zero for not directly engaging them - it's an exertion of power - not because they can't easily figure out where Nintendo ranks in the spectrum. From Nintendo's perspective - complying with all these NGOs is fine for companies that have 50,000 employees - but Nintendo would have to dedicate a giant chunk of its workforce if the flood gates opened - for something they feel they already do an adequate job of since they work with certified suppliers.
Aside from that, one need not like the NGO in question to actually care about conflict minerals. Please don't conflate the two. There are lots of terrible, self-serving, and ethnocentric/Eurocentric NGOs out there with highly flawed ranking systems - I witness as much in my parents' country of origin which I visit on a yearly basis. That doesn't mean we as society should not value the implications of our actions, and understand there are consequences of our consumption - including our ability to influence it. Just look at Apartheid and how boycotting companies doing business there raised awareness of the issue and helped as a catalyst to end that racist regime. Look at all the talk about BDS right now - with the New York Senate stepping in to punish participants in that boycott because they realize the implications if a full boycott were to materialize. Even if you choose not to do anything - don't actively dismiss these issues as pointless or unworthy of attention - there are actual human lives at stake!
That said - for those of you who are interested in applying technology to supply chains - check out this project started at MIT -
http://www.sourcemap.com - really cool and has the potential to really help small companies comply without hiring hundreds of people and spending millions on audits which certain NGOs demand (sometimes the NGOs themselves are the only ones that can be hired to do the independent audits to get the highest scores - so some of these ranking systems have corruption and self-enrichment built into them).
Just my 2 cents.