jsmn is great if you use it as a lexer and you know that your input is well-formed. We use it for our games to read in our configuration, gamemodes and level files. We've just built a few wrapper functions around jsmn and called it "jsmx", and it suits our needs perfectly :-)

But you're right, if you want a json library to parse arbitrary json input using a more complex library like this one is the easier and more viable route than using jsmn.