"Shellfish attitude" sounds like "selfish attitude". A selfish attitude is when one is concerned excessively or exclusively with oneself. Shellfish are a type of aquatic invertebrate that can be consumed at restaurants (these invertebrates include lobster, crabs, clams). The person getting referred to in the joke is a cook. Cooks can create seafood dishes with shellfish. This joke is a play on the word "shellfish".
Some cooks have a shellfish attitude .