We'll probably never know the answer to that question, as this isn't an invention in the usual sense. Like most ideas it was probably thought of by different people, with each one adding a bit on, and then suddenly went viral.

It is basically a variation of an idea that has been around ever since the word 'thermometer' became a household term. While the thermometer itself is an old invention, it wasn't until the 20th century that almost every home began to have one: And it's in the 20th century that you start getting these 'o-meter' words. The earliest I can find is 'clap-o-meter' to measure audience applause, which was around in the 1950s and possibly earlier.

Thumb-o-meter is quite recent - it's become popular only in the last couple of years. While it's usually used to indicate how you feel about something, one gardening blogger has cleverly adaped it to show the health level of a plant - a good example of how these new words can be used in lots of ways and don't really belong to anyone.