Most supervillains, especially the genius inventors, should be able to make a perfectly lucrative and legit career. Some eventually do, while others finally admit it's not about the money (and probably never was).
I have to take issue with the Mysterio example, though. His origin used to be that he was fired as an fx man, showing that it's quite possible he simply couldn't make it legitimately for some reason. Also, the market for practical fx shrinks every day. (In fact, having him turn villain as frustration against the cgi field might be a good modern interpretation.)