Quite the opposite, lower means worse if the sync rates haven't changed. SNRm is effectively how much spare capacity there is on the line to account for the natural changes in line condition due to the weather changing the ability of the copper to conduct the signal, or any temporary radio frequency interference.
But given the above, its expected to fluctuate, it exists purely so that it can without losing sync.
DLM works on the basis that the less errors it sees, the less the SNRm fluctuates, the lower it can set the target to obtain a faster sync rate. So the only time SNRm being lower is better, is if you had an increase in sync rate accordingly, as then it suggests DLM has deemed the line stable enough to have a smaller safety margin.
