Pop-Rock Stevie Nicks: ''The Wild Heart.'' (Modern/Atco Records 7 90084-1.) It's Stevie Nicks's second solo album away from Fleetwood Mac. How's she doing? Not too shabbily, judging from this effort, but her music is in need of more outside help. Though the song patterns (she wrote or co-wrote all but one of the songs) and vocal styling become a bit tiring if this record is listened to in toto, this is an engaging set of love ballads. The duet with Tom Petty and his Heartbreakers for the Petty-written ''I Will Run to You'' was a welcome change, though hardly a break from the generally somber tone of so many of the cuts. Petty & Heartbreakers' spare guitar plucking and hoarse-sounding vocals become somewhat the centerpiece of the disc, coming, if you will, in the middle of the set (the second song on the flip side) and breaking with the somewhat similar musical patterns of the rest of the music. (Many of the songs sounded as if written to showcase Ms. Nicks's sometimes thin-sounding voice.) There's some rousing material here - like ''Enchanted'' -but not enough musical imagination. Still, for Nicks fans -and others who enjoy listening to the Fleetwood Mac school of music -this one should satisfy.