He started out as a "Gypsy", and his family was dirt poor and his mother a witch. That right there is problematic because it feeds into the stereotype that Roma are trash eastern mystics.
In some versions of his origin, he then spends his youth, in part, stealing, a "gypsy" thief.
Even in versions that don't include stealing, he spends his youth as a terrorist. And since Roma are originally from the same part of the world as Indians and Pakistani, this is problematic.
He is from Eastern Europe and of more purely Eastern blood, and is constantly on about honor. Portraying Easterners as all about "honor", and "honor" being treated as a barbaric form of morality compared to Western morality is an old, problematic trope, and Doom is steeped in it.
Finally he's dressed in armor that evokes a Soviet design, with all its little rivets and grey steel. Combined with his general contempt for democracy, it screams of outdated Eastern European stereotypes.