[@Drakey]
All Titans are connected to their pilots on a mental level. 

Rewatch the moment when Jack first got into BT

And putting heavier armor on a Northstar just counteracts the entire point of its agility: the Northstar has a lighter frame than medium and heavy titans, and for good reason- its carrying less, so it needs less. When you put more armor onto a frame designed for speed, you seriously compromise that speed. Your Northstar would be able to get run down other medium titans, and definitely by other lighter titans. And the extra armor wouldn't really save you from that.