Particularly disease-carrying mosquitos have been assessed to be unimportant to ecosystems. Although, it's worth noting that outside of those few species, they don't primarily feed on blood, but rather nectar. They take blood once during their reproductive cycle.
Are they? I thought I'd heard this in an African context specifically. The jist was that there's endless roughly equivalent species that don't do multiple blood meals, which would basically just take their place for everything but spreading pathogens.