Poor people using spices to cover rotting food is a complete myth falsely attributed to the Middle Ages. Spices were incredibly expensive and a luxury limited to the upper class - anyone rich enough to afford spices did not have to worry about rotting food.
The actual reason for the perceived blandness of White American food is basically the converse of this. Stereotypical white suburban food is probably closest to mid western cuisine.
The mid west was uniquely isolated from Spanish, French, or Italian influence (which were heavier in tastes), lacked international trade to get any spices, and as the nation's bread bowl specialised in and received lots of subsidies for growing staple crops, like corn.
Ethnically, its white populace is overrepresented by more Anglo ethnicities, like the British, German, and Nordic, which also had more, shall we say, limited palates.
Spices Were Used to Mask the Taste of Bad Meat in the Middle Ages? - https://culinarylore.com/food-history:spices-used-to-cover-taste-bad-meat/