I have fond memories of this. The friendly mouse and dog team up to deliver a letter to Santa. They go all over town but keep running into store Santas or Salvation Army types or whatever. There are the usual Hanna Barbara songs, voicers (Daws Butler and Paul Winchell), treacly images of an ideal Christmas, etc.
And know what, I loved every single second of it. They'd put this on about 7:30 pm in December back in the 70s, and I can remember playing outside in the snow as a kid, coming into the house and seeing this come on. It was a perfect tandem show w/ the Grinch/Rankin Bass/Peanuts stuff that is now better remembered.
It's not a classic per se, but its not that bad, and if you ever get a chance to watch it, do so. You gotta like H-B from the 70s though. Fair Warning.