Seems very 2000-ish to me. The dreamcast was the first console to operate at a decent resolution but didn't have the memory or processing to do games with a lot of texturing so often had a lot of bold colours in its games with texturing more limited. Splatoon has that look because Nintendo themselves love those type of graphics its part of their house style. They don't do realism. Splatoon most looks like Mario Sunshine that seems to be the game that inspired it.
https://wiki.dolphin-emu.org/images/d/dc/Super_Mario_Sunshine-Goo_Dolphin.png








