The Constitution guarantees a trial by jury, not any sort of further representation. Court appointed attorneys are a protection given by the state, and the state has no obligation to provide anybody with a certain (very subjective) quality of representation. Court appointed attorneys are far from incompetent, and the vast majority of court cases are fairly procedural - the lawyer doesn't particularly matter.
Ultimately the justice system comes down to one simple point: Someone is on trial and they are either guilty of what they're being charged of or they're not. Although the truth doesn't always come out, a "good attorney" isn't someone who gets a guilty person off, and in that sense there's no reason a court appointed attorney would be "inadequate" to defend someone. If they're guilty, there's no adequate defense regardless of who the attorney is.