Why not? Emilie de Ravin is a natural blonde that had to dye her hair brown for the role of Belle (Despite that Emilie looks better as a blonde). Why shouldn't Sarah Bolger have to dye her hair blonde, or at the very least wear a blonde wig? She actually looks really good as a blonde.

I didn't really mind the brown/red hair but it would have been a nice touch for Aurora, since she's my favorite princess since childhood and besides it suits Sarah very well.

The only thing that bothered me was how inconsistent they were with the hair color: when Maleficent cursed her, her hair was dark brown, but when she woke up it was much lighter; and then in S3 her hair was red.

She looks gorgeous anyway though.

It doesn't bother me too much. I mean Once Upon A Time has so many deviations form the Disney versions so what's one more?

Yes

I think Belle is more famous as a brunette than Aurora/Sleeping Beauty is as a blonde (for me, anyway - it probably depends on what Disney films were out or popular when someone was a child), so it made more sense to have Emilie de Ravin colouring her hair. Have there been other non-blonde adaptations of Aurora/Sleeping Beauty?