Yes, in order to dress your Sim baby in clothes of your own choosing, the originals must be forgotten.
There is really only one choice if you want to move away from those monkeys and butterflies, and that’s to find custom content to override the defaults. Who doesn’t after a while? Where’s the individualism, right? In fact, you’ve probably gotten sick of seeing those same darn outfits over and over. Green onesie with a monkey for boys, yellow onesie with a butterfly for girls. If that’s not sad enough, they are also permanently in one outfit. Technically they’re objects – very needy ones, but objects nonetheless – and as such are forever tied down to their bassinet.