Having the 'Show HN' posts all tagged with platforms would sorta make me feel like I'm in a supermarket, skipping past the isles containing stuff I'm not going to walk out with.
Why all? My estimate is that most of them are web apps, mobile apps are often at least Android/iOS, I would say that those tags I proposed would be needed quite rarely. And it's not like I am demanding it, it was just a proposal to see whether others would like or not - just my $0.02 - you certainly raise a valid point, we certainly do not want to complicate things too much.