Yeah, it's heckling, but I think it's a little weird to discount what the guard's heckling him about, i.e. his racial make up. Even if you restrict heckling to just looks, there's plenty of things the guard could've chosen, the most obvious being levels of attractiveness. But the guard chose race.

More importantly, you have to ask yourself why if it's "just heckling", we need to see the conversation. What's the importance of it? Hint: the importance is that Gaiman is showing us obliquely what Shadow looks like. There'd be no need to include random, trivial conversations, otherwise.