They are round just due to gravity. The larger the mass the more likely it is to be closer to a sphere - that's why smaller rocky dwarf planets aren't often very spherical. Their mass isn't quite enough to conform them to a sphere.
Yes extreme rotation can make them oblate - especially gas giants.
Here is the definition of a dwarf planet... Note the comment on mass and roundness.
According to the International Astronomical Union, which sets definitions for planetary science, a dwarf planet is a celestial body that -orbits the sun, has enough mass to assume a nearly round shape, has not cleared the neighborhood around its orbit and is not a moon