ROCK/POP Billy Squier: ''Emotions in Motion.'' Capitol. ST-12217. From the sound of several of Billy Squier's songs on this LP, rock stardom isn't easy. ''Emotions in Motion,'' his third solo album, has images like ''nights of confusion and impossible dreams/days at the mirror, patchin' up around the seams'' from the single ''Everybody Wants You.'' Squier paints a convincing picture, autobiographical or not, of overactivity and frayed edges, hypocrisy and the sacrifice of a wholesome life style. There are other themes as well, the obligatory ones, for instance, about searching for love - all presented with Squier's powerful, expressive voice and his versatile guitar. Mr. Squier's success is due in great part to the single ''In the Dark'' and the album of strong songs which it spearheaded two years ago (''Don't Say No.'') There's nothing that quite matches ''In the Dark'' as a song or arrangement on this LP, but ''Dark'' was an accomplishment that would be tough to equal. Squier wrote all the songs and does most of the vocals and guitars for ''Emotions,'' a challenge requiring talent and imagination. For the most part, he pulls it off with an LP full of his own brand of rock. Occasionally, one feels he could have gotten a boost - a feeling of more well-rounded creative vigor - from some outside help. Nonetheless, ''Emotions in Motion'' is good rock by one of today's most distinquished musicians.