"A lollipop in a hamburger" can be understood as putting a lollipop in a hamburger, which is an unreasonable combination, because lollipops and hamburgers are two different types of food. However, this combination is expressed to emphasize a feeling of enjoyment.
"It's cool" is used to describe this kind of imaginary enjoyment. It may be a colloquial expression, which can be understood as "that's great, very comfortable and very enjoyable".
Generally speaking, this sentence may mean to express some imaginary enjoyment, such as eating hamburgers, if you put lollipops in them, it will add more taste and fun. However, it should be noted that this combination is not practical, because lollipops and hamburgers are completely different foods.