I'm with Jerry all the way on that. Being honest means singing songs that speak to what you are concerned with. They may be songs you have heard from other people, or found for yourself, or made up yourself, but there are reeasons why we sing some songs and not others. (And most people have to write a good few songs that don't work too well before we learn how to write songs that do work. And if we don't sing songs in public we'll never learn the difference.)

At different time in our life we have different things we are concerned with. A young person may be thinking about love and rejection and insecurity and stuff like that, an older person will be thinking about time passing, and sooner or later about dying and about the good old days and so forth. (And probably also about love and rejection and insecurity too, but maybe a bit more circumspect about opening up about stuff like that.)

As for mumbling, you have to crawl before you can walk. Mumbling is one way of hiding a little as you open yourself. But singing out a little bit too loud can be another. So is playing finger-breaking music that doesn't get anywhere. We all put on fronts from time to time.