I used to take 1 multi-vitamin before pregnancy till now.
Balance diet is good for both you and your baby since supplement provides vitamins and minerals ony; no protein or carbohydrate available.
If you have too much junk food, your milk will become very oily and low nutrition. so your baby can hardly have enough protein to grow and energy to play. in addition, your baby may intake too much 膽固醇; this is no good for your baby too.
besides, you may consider taking omega 3 to increase the DHA in your milk.