The slope of the scales has no bearing on whether percentages are meaningful here. The problem with both systems when it comes to percentages is that neither system has 0 set to a natural 0. This leads to an entirely arbitrary point on the scale where decreases in the unit will approach a 100% difference and then suddenly start decreasing again.

If anything Fahrenheit should be less insane because at least the artificial 0 is likely to stay much further away in the data they're quantifying so the percentages stay reasonable.