A 30% approval rating means ~75-80% of Trump's base is still fully behind him. A two-party system will only very rarely produce figures with over 50% approval, because roughly half of voters disapprove on GP. The apathy of the 40-50% who don't vote is also a product of a system that only allows for two choices and not something to blame the apathetic for. It really makes no difference whether they vote or not, because there are only two choices and neither represents them.