The thrust of "Stuart Little," the first movie, was how George and Snowball came to accept Stuart into the fold; the lesser but still appealing sequel revolved around Stuart wanting some companionship on his own level. The series doesn't have any such conflct, with the Littles (and Snowball) now one pretty contented family, and mired in incredibly bland plots (going to the carnival, spending the night at a "haunted" inn etc).
And "bland" is the word; bland writing, bland animation (and is it just me, or does the premise of a human family having a mouse as an adopted son lose something in full animation, where such things are more commonplace?), bland voice work - even allowing for Quinton Flynn as Snowball, David Kaufman standing in for Michael J. Fox as he did with the animated TV series of "Back to the Future," and Hugh Laurie returning from the movies as Mr. Little - bland everything...
Like Mr. T and Chuck Norris with their cartoons, the "real" Stuart (c/o Sony Pictures Imageworks) appears at the beginning of each episode to set up the story and at the end to tell us the moral, but it only makes the gap between the movies and the series even wider; the charm and interest of the big-screen movies is gone, replaced by something closer to "PB&J Otter" in overall effect. It's not awful - just very, very mild.