Depending on the size of the lobster rolls and what ingredients are in the lobster rolls they typically contain between 600 to 1440 per lobster roll.
A plain ole lobster roll would contain around 600 calories so they are not that fattening unless you eat a lot of them.
I worked on losing weight and I still consumed a lobster roll every few days and I still lost the weight I was working on losing.
So you could still eat lobster rolls in moderation while losing weight and as long as you exercise and eat other foods that contain less calories you should have no problem losing weight.
The cooked lobster only contains around 83 calories so most of the calories in a lobster roll come from the other ingredients in the lobster roll.