I am breastfeeding - does what i eat affect the nutrients the baby gets in breastmilk? for instance, if i eat something less healthy like pizza or a cookie, will the baby get that through breastmilk and be less healthy?

The breast mechanism sources the nutrients the baby needs from your body, either from the diet you have or by drawing from the rest of your system/bones/etc. A poor diet does not replace the nutrients the breast draws from you. In the end, baby will fare better than you from your dietary choices, but an occasional lapse to enjoy junk food will not hurt it.

Your nutrition does affect the quality of your breast milk however it is not as simple as you are making it. Eating pizza and cookie occasionally does not hurt anybody and would not hurt your baby.