Isn't it overkill to use a regulated power supply for a nixie tube?
Sure, it would work. 

I have used a slight step-up transformer (120v to 130v) to drive a nixie supply. It also had a 6.3v filament tap which was really convenient for the 5v ttl logic supply. However, I used a full-bridge rectifier followed by a electrolytic capacitor to filter both supplies. Here's why I would recommend that setup over what you're suggesting:


It's cheap. For $1.25, you can have decent regulation of your supply voltage. I'm assuming the isolation transformer is already accounted for.
You can/will see the nixie tubes flicker at 60 Hz.


The motivation for fancier regulation circuits is to protect the nixie tubes. They are usually the focal point of any design using them, so it's in the designer's best interest to make sure nothing bad happens to the tubes. I've seen people drive nixie tubes with expensive regulated bench supplies, and read about people using unisolated circuits. Somewhere in that range is a cost-benefit-paranoia tradeoff.