Output power (mW) is power that the modem is using communicate with the DSLAM. The lower the value, the better it is. Typically the closer you are to the DSLAM, the lower is the power required, which is what is in your case. Is it actually "0", no, the value may be too low to be reported, or in most cases its just the modem can't read the value correctly and reports it as "0".
The line stats otherwise are excellent. No need to loose sleep over it.
