Choosing a certain set of clothes makes no difference on whether you get the bonus star/heart. If you press the hanger icon, you see your star and heart points on the left side. If the points are high enough, then you'll get the bonus star or heart. Some ways you can raise these points include buying clothes and buying furniture.
When you begin a date (and a project, as well), you'll see a number such as 123/456 in the bottom left corner of the screen. Apparently, the bigger number is the max amount number of hearts (or stars) needed. The smaller number is where you are currently.
Comments
It's been bothering me for ages now I get it )