Other than a possible issue with how sugar is processed, many Duncan Hines frosting flavors are a-ok for vegans.
Sugar is an issue for some vegans because some refined sugar is processed with animal bone char, although this is not stated as an ingredient in sugar.
The classic vanilla frosting flavor contains sugar, vegetable oil, water, and corn syrup, but no milk products. On the other hand, vegans generally have to be wary of homemade frostings, which may contain butter, milk, or heavy cream.