If a parent has curly hair and another parent has straight hair and the child has wavy hair is that incomplete dominance

No is not The xy chromesomes may have stronger or even genes therefore the child has a mixture of hair .the roots may be straight but the ends may be curly

Codominance is when alleles have the same amount of dominance and both are expressed in a heterozygous individual(ab blood type, roan cow). in a heterozygote with incomplete dominance, the phenotype seen is an intermediate of the dominant and recessive phenotype.(pink flowers when the dominant form is red snd recessive form is white)