Bad question. Instead of seeing seasonality as the dominant factor or a simple detail, it should be considered as a variable whose impact on the travel budget depends on flexibility.
It is the traveler's flexibility that determines whether seasonality is an insurmountable wall or an open door for significant savings.
It is neither one nor the other in absolute terms; its influence is strong, yes, but mostly conditional.
For example, a retired couple without date constraints can aim for the low season for much lower airfares and accommodations, while a family with school-aged children has no choice and will always pay full price in the middle of summer.