Her dad had a pretty big nose, if we want to go there. Nothing is said on the Wikia. And this question doesn't really matter... but I'm sure people want to know! Was Miyazono half-Japanese? What indications are there that she was?
There is no evidence to suggest that Kaori is half-Japanese. I discuss my reasons for this assertion below.
The fan wiki for Your Lie in April makes no such assertions about the ethnicity of Kaori. Were there an indication or (obvious) suggestion that Kaori is half-Japanese in official material, I would expect someone to have noted it.
Kaori's parentage is straightforward, and her parents have Japanese names and do not look overtly foreign.
We cannot extrapolate any information about ethnicity from the characters' hair colours. While Kaori had blonde hair even as a child (as seen in the OVA), so did Takeshi. Tsubaki already had brown hair (rather than black) as a child. Takeshi's sister, Nagi, is also blonde. It is difficult to imagine primary school students dyeing their hair, but it is also difficult to imagine that all of these characters have significant non-Japanese heritage. It is best to chalk this up to a lack of realism in anime hair colours.
As far as I am concerned, I would thus err (at the time of writing) on the side of assuming that Kaori is not half-Japanese. Were Kaori half-Japanese, it would seem to be such a large character detail that it would have to be disclosed somewhere.