Good question. I encourage additional clarifications but here is my opinion of the term.
A SoftModder is anyone who enjoys making modifications (or changes) to their phone. This includes simple physical modifications, like adding accessories; minor changes, like personalizing the display or more advanced modifications which require the phone to be "rooted."
Basically, anyone who wants to improve the performance of their phone, unleash additional "hidden" capabilities and/or pimp their device is a SoftModder.