Pop-Rock Robert Plant: ''The Principle of Moments.'' (Atlantic 90101-1) - This LP echoes strongly of Led Zeppelin, despite Robert Plant's attempts to detach his present musical efforts from the backdrop of his former group. In large part, it's because of his distinctive voice, sometimes raucous, sometimes soothing. But it can also be attributed to the way Mr. Plant writes his music, as it embodies the same somewhat astringent sound of the days of Zeppelin. This record's main flaw is its dependency on keyboards - which often muddy songs that could have succeeded. Fortunately, there are moments of escape when the crisp, undistorted guitar work of Robbie Blunt emerges. Plant's vocals on the album are clear and urgent as usual, with a desperate tone in several songs. There is a pleasant surprise in ''In the Mood,'' a bouncy yet slow-beat rocker with very simple subject matter, dancing and singing. The song has a fine hook, and the guitar work is uncluttered and effective. Nonetheless, Plant will need to break away from the heavy sound of his former band to attract new listeners.