Yes and even in most of northern Taiwan, there's a ton of Taiwanese everywhere. One of my old friends was born in Neihu and moved to the US around 3rd grade or so. When he visited me in Taipei as an adult, he spoke fluent Taiwanese and much more limited Mandarin. His situation was a bit comical to younger people, but not a real obstacle.

I also lived in the Guishan/Linkou area for a year and heard a lot more Taiwanese than Mandarin in day to day life.