Shouldn't this be a concern based on how the culture in question operates, and/or how the character would feel about it, rather than a generic concern? If not it seem, ironically, like a form of American cultural imperialism (not an uncommon kind - I've seen all sort of wild assumptions about how societies outside the US work based on US models which don't apply, from even most good-hearted of people). It's not like all the cultures which use multi-syllable names are non-white (quite the contrary, in fact), nor are all cultures which shorten names white, is it? Obviously the Vistani have historically been a Roma analogue - do the Roma use solely multi-syllable names and not shorten them? I actually don't know. There's also the question of whether they remain a Roma analogue in this version.
It's also worth noting, that if you travel the world, you'll find most cultures (of all ethnicities) modify your name, whether you like it or not. It's curious that you seem to think only white Americans do this - go really just anywhere else in the world and see what happens to your name. It is unlikely to remain static, esp. outside of an English-speaking country. I actually agree that it can be a racist microaggression, or even an outright aggression (racist or otherwise), particularly when some smarmy git knows perfectly well that you are called one thing and they shorten anyway it (often in a way that's faintly insulting), but it seems like you have to pile up quite a lot of assumptions to get to Ezmerelda being know as Ez as racist (at the very least that the Vistani are still Roma analogues, that the Roma prefer multi-syllable names, and that is unlikely Ezmerelda's character would instigate the usage of this shortening).
