Casa Mare is often really packed. And has been for decades. That has less to do with the food, which is not that special in any way, than the neighborhood it is located at.
Lauttasaari is a suburb with a clear identity because of its size (close to 20,000 people) and geography (it is an island). That leads to it being quite self-sufficient - one junior high school, one high school, one big gym, one big basketball club....you get the picture.
For like 30 years, Casa Mare has been "the place" where people meet. For a local, it is not uncommon to see a few relatives, neighbors, hobby friends and maybe even work friends each time going there. Further, a few years ago, the restaurant also rented the neigboring premises, enabling them to offer high practical meeting facilities for the local organizations (which are many)
So not a place for a tourist to visit. At least not for the food, but maybe for anthropological studies on what regular Finns look like.
Ed note: I no longer live in Lauttasaari. But I have lived in 7 different addresses there so I know the scene. And still return to Casa Mare, time to time.