Is using a ground AND supply pours on a 2-layer PCB a bad design decision?
No, it's not a bad design decision, but it's often avoided because it can be a bad design decision sometimes, and many simply choose to use only ground pours on outer layers of the board regardless of the number of layers.

A lot of engineers choose to use only ground pours on outside layers for reasons of impedance (signal integrity), because ground is needed more often than power, and because it's common, many simply assume any visible pour is going to be ground. Many put power planes internal to protect them from damage - an errant screw rattling around inside a case coming into contact with a ground plane is less likely to cause a problem than one coming into contact with a power plane.

It's very convenient to have power pours, though, for some designs, and in two sided through hole designs it does make things easier in some ways. Once you run into a problem, though, you too may adopt a "ground planes or no planes on the outside layers" policy.