Mountain Dew and Twinkies are bad for your health regardless of your income level. We should tackle unhealthy eating by going after the supply, not by going after a class-segmented group of consumers.
Like many Americans, I grew up in a town where unhealthy eating was a major part of the social rhythms of life: a bag of buttery popcorn at the movie theater, an ice cream at the zoo, things like that. Not having the means to participate in these simple pleasures is a kind of social deprivation. I view redistributive programs as a tool to lessen the gap between families. Food regulators can handle the junk food problem.
The moral calculus is not the same.
I don't think we have an obligation to legislate everyone's health, but I do think it's a higher ask when we're talking about explicitly subsidizing bad choices for people most vulnerable to making them. I don't think we should subsidize cigarettes for poor people, either, even if that means they are still accessible to rich people in a way that's perceived as unfair.
And besides: people of high incomes already disproportionately avoid these highly processed foods, so it's not like we're hoarding the wealthy pleasures of Mountain Dew and Twinkies just for them.