Indeed, all the shutdown polls really show is that how you feel about Trump and who you blame for the shutdown amount to the same thing. If you approve of Trump’s job performance, you very likely blame Democrats. If you disapprove, you very likely blame Trump or congressional Republicans.
Across six polls, Trump’s net approval rating (the percentage of people who approve of the job he’s doing minus the percentage who disapprove) was always within a few percentage points of the net blame difference (the percentage of people who blamed congressional Democrats minus those who blamed Trump or congressional Republicans). The average difference was just 3 percentage points.