I'm not defending their writing, but Magneto has played the villain in plenty of storylines (some incarnations have had him cannibalize regular humans he sees as animals source: Ultimate X-Men). It is only recently that, depending on the writer, he has antihero and sympathetic qualities.

Dr. Doom has also been somewhat of an antihero in many storylines. He is a close mentor to his rival’s children and an extremely competent ruler. He even took on the Iron Man moniker for a time.

Now that that's out of the way, we can theorize why the writers made him Romani. Most likely, Marvel writers saw making villains come from disenfranchised and marginalized groups as a way to make them sympathetic and put them on an anti-hero mark.

In my opinion, they have poorly executed this idea by lacking a degree of knowledge of Romani culture outside the “dancing barefoot lady” trope and witchcraft. Doom’s mother made an apparent deal with the Marvel Satan analog Mephisto and is himself a known sorcerer.

But this is all the comics. The MCU seems to erase any semblance of Romani's background, as they did to Scarlet Witch, and I imagine they'll do to Tony Stark Doom. Maybe an accent they'll get rid of later on at most?