The awesome Aliette de Bodard (herself of mixed race) has some scathing things to say about the portrayal of mixed race people in fiction. She points her finger at lazy writers who use tired tropes where mixed race is a shorthand for any number of maladjustments. That this is simply offensive should be no surprise to anyone but apparently it is. Why anyone would assume that being of mixed race will inevitably lead to alienated and damaged people from malfunctioning families is hard to believe.
Racism certainly can be a problem - and we obviously have a long way to go before that nasty aspect of humanity is overcome - but why not acknowledge that mixed race can be just as much a positive as a negative? The truth is that writers who use mixed race in this way are writing characters who are little more than caricatures, cardboard creatures they do not allow to grow and develop. People are complex beings and any characters, to be satisfying, should be the same.
We've all read stories where we have the stock standard range of characters - the jock, the nerd, the beauty queen, the wall flower, the brooding hero etc. These characters never truly grow but these are stories working to a formula where the reader knows what to expect and would be disappointed if they deviated. I don't think we should pretend there's not a place for these stories. Many folk like reading them and they don't claim to be anything other than what they are - a piece of wish fulfilment.
But if you are looking for more as I am - a deeper exploration of the human state and understanding of people, what makes them what they are and how they can change as they learn from their experiences - then you must move beyond stereotypes and lazy tropes.
BTW the comments are also illuminating and worth looking at.