Whether a majority agree it’s a serious problem depends on whether you think “somewhat serious” is “serious” or not. If I answer there are “somewhat serious” problems, you can’t just round that up to “serious” because that word is present.

If the categories were “serious, moderate, mild, and not a problem”, which I’d argue are semantically equivalent to the survey categories, then it’s clearly an error for the headline to be written as “majority agree income inequality is a serious problem.”