I had some trouble with this trophy because the sex appeal bar was moving from 0% to 45% randomly, no matter the clothes or the car, just didn't get the 100%. So looking over some forums i found a glitch to get the trophy in early game, but I'm just about to finish it and buying clothes in DS didn't helped either and haircut did nothing as well.
If you're having the same problem, this is what I've done:
Go to the safe house near the driving school
Get totally naked, just underwear.
I made a new save here.
Get any car parked in the driving school parking (Bullet worked for me).
Go to the tattoo shop in Hashbury (park the car in front of the door) and remove all the tattoos you have. Look at the % of sex appeal, should be low.
Now buy every tattoo in the shop, if there are 2 types, buy the expensive one. The % should be higher after this.
You can look at % sex appeal without getting out the shop. (If you press options button and go to stats look how much sex appeal you have)
Now go to the "Lower back" tattoos and buy the other one you don't have, then buy the first one. Do this a couple times and look at the stats, the sex appeal should be higher. No need to remove tattoos, just keep doing them and replacing the other.
When you hit the 50%, go outside the shop and the trophy should pop even without getting in the car.
Hope it works for you as well.