Gold Stars shown next to clothing items indicate how much they will increase your character's Professional Bonus for projects. Having enough Gold Stars will grant your character a bonus star when you begin a project. This means you only have to fill 4 blue stars instead of the usual 5, saving you energy and time.
The Gold Hearts on clothing work the same way, increasing your Romantic Bonus for dates. When you have a high Romantic Bonus, you'll get a bonus heart at the beginning of your date and will only need to fill out 4 of the 5 hearts to complete your date perfectly.
You can avoid having your date insult your attire by ensuring you your Romantic Bonus score is as high as possible. Your current Professional and Romantic Bonus scores are displayed to the left of your character in the wardrobe. You can improve these by purchasing items in the wardrobe and in homes which have a star or heart symbol on them.
Please note that you do not need to be wearing specific clothing in order to get the bonus rewards; simply owning the items will contribute to your overall style points!