Pop-Rock Elton John: ''Too Low For Zero.'' (Geffen/Warner Bros. GHS 4006) - Elton John has put his eccentric flamboyance on a shorter leash and has produced yet another LP of splendid pop material. Exceptional pop songwriting, competent - sometimes pretentious - vocals, and fine arrangements make up some of the British singer-pianist's best work. The song title ''I'm Still Standing'' could be emblematic for John this time. His halcyon days of the early- and mid-'70s are almost forgotten by younger record buyers, nonetheless he is producing music that matches in pop quality his earlier songs. ''Too Low for Zero,'' in fact, is a singularly odd selection for the title song - it being about the less than transfixing subject of someone having trouble sleeping because of his personal problems. Nonetheless, it lets Nigel Olsson pepper the song with excellent drumming and affords John a spot for some crystalline piano playing. It's the rockers ''I Guess That's Why They Call It the Blues'' and ''Kiss the Bride'' that have and will get the most play. They're not hard rockers, mind you, like some of Elton John's earlier work (''Saturday Night's Alright for Fighting''), but then this musician's best successes came from the milder stuff anyway. Stevie Wonder's harmonica on ''Blues'' sings splendorously. Tooted as the reunion album between Elton John and Bernie Taupin, this is a better side of Bernie (Taupin did write the lyrics for five of the songs on Elton John's last album, ''Jump Up''). His words aren't too maudlin, or coy, as they've been in the past. This LP also marks Elton John's judicious use of synthesizer; he had left that responsibility with another on his last album.