Few, if any, will ever wear a tee shirt unless they like the art, and/or relate to it's slogan- if it HAS one.
Sometimes, a catchy phrase ALONE will 'sell' the shirt- but not often- and even TEXT can be enhanced with an artist's touch.
      Artist's, as with cooking, have to choose the right ingredients, and 'cook' them properely using the best choice of subject matter, fonts, and colors.
      The client can hinder them with a tiny budget, or help them with a larger one. By giving the artist a Starting Point- BOTH are helped.
      Since a Tee Shirt is virtually a walking billboard, investing the right time and budget into producing it will bring the best results.