How old are they? (Or if you do not still have them, how long did you keep them?)
I have seen some chicks that grew barred-looking feathers first, and later grew laced feathers as they got older.
And purebred Cornish do grow slowly. They also have close-fitting feathers, so they look smaller than other chickens that are the same weight. The common "Cornish cross" meat birds that grow so fast originated as a crossing with a White Rock, to get them to grow faster.
If they were still mis-marked when they grew up, and never grew to an appropriate weight, then I certainly agree that is a problem. But sometimes chicks go through weird stages as they grow, and turn out fine when they mature.
Not from McMurray. They offer the standards but not the bantams.