。
It's as smooth as a baby's bottom.
Literal
Baby [attributive-の] bottom [like-みたいに] smooth-smooth is-[copula-だ].
A stock simile — 赤ん坊のお尻みたいに ('like a baby's bottom') is the conventional Japanese comparison for smoothness (just as in English). すべすべ is a 擬態語 for smooth, silky-to-the-touch texture — common with skin, leaves, polished stone. みたいに is the casual equivalent of のように.