You may be confusing mRNA and tRNA. tRNA it's used by ribosomes during the synthesis of DNA. mRNA is created during transcription and exists in the cell for only a short period of time before it is degraded. During its lifespan, it may be translated into protein by ribosomes. During the lifespan of one DNA, countless mRNA transcriptions will take place. One erroneous mRNA would lead to one malfunctioning protein before the mRNA breaks down, whereas one erroneous DNA would lead to countless erroneous mRNA, leading to countless malfunctioning proteins.