I like that they gave him Rainbow hair, because it lessens the association that it, and by extension Rainbow Dash, has had with the lesbian community. They could have kept it straight, though, and had the mother with the ruffled hair, so that Dash looked more like a combination of her parent's traits. Not biology, I know, but it's how it happens in cartoons.

What's wrong with the implication is that a character who is not called a lesbian is being widely thought of as a lesbian simply becasue of her hair color.

It's a flawed association to make, and might not reflect the realities of the situation. There is nothing wrong with rainbow dash being a lesbian, there is something wrong with rainbow dash being called a lesbian only because of her hair color.