POP/ROCK David Bowie: ''Let's Dance'' (EMI ST-17093). Bowie's new album is a pop success in many ways, but not a musical one. Apparently to boost record sales, he has dropped his earlier standard of innovation. His music here doesn't explore metaphysical and existential realms. ''Let's Dance'' consists mostly of Euro-pop dance music accompanied by lyrics so accessible they seem almost mindless. The overall sound, made distinct only by Bowie's voice, is musically simplistic -- lacking his previous instrumental and percussive intricacies. ''Moderm Love' is the most evocative track. Bowie's concise words express a fear and loathing of the plastic state of contemporary romance, Ironically, the music , tho tight, is ridden synthesizer effects that make the song sound as synthetic as the conditions which induced Bowie to write it. Meanwhile, ''China Girl,'' a combined effort with Iggy Pop, is cloying in its Oriental overtones, and the title track, ''Let's Dance,'' is overlong, repetitive, and lyrically bland. Bowie's songs here bend toward the public instead of commanding people to ''turn and face the strange'' as he once did on his ''Changes'' LP. Although Bowie effectively incorporates sounds of the '50s and '60s music in his new work, he contributes nothing new. While one can't condone Bowie's decline in original thinking, his courage to try to reach the general population with music that is palatable can be admired. Yet 'Let's Dance'' is like a poet laureate attempting to speak in the.idiom of the masses. There's nothing wrong with that -- Wordsworth did, didn't he?