ROCK/POP Joni Mitchell: ''Wild Things Run Fast.'' Geffen Records: GHS 2019. Joni Mitchell mixes rock, folk, and jazz for a stylish stew that's all her own. Her cool-sounding, somewhat detached singing matches the mood of her paintings - which decorate the album. For the most part, this is an album of love songs, but they don't become tiresome as does the music of some performers who concentrate mostly on this one subject. Some songs work better than others, of course, but each has its own accomplishments. This listener particularly liked the jazzy ''You're So Square,'' the only song not written by Ms. Mitchell, and the energetic ''Solid Love.'' This LP is remarkable for its faultless blend of fast and slower music, and it's breezy, yet thoughtful sound.