Edit:
I would be less not ok with fines that scaled by income if they were reasonable for the infraction. Fines in the US are not reasonable (with a few state by state exceptions) unless you're upper middle class. If we scaled fines in the US so everyone got screwed as hard as poor people get screwed that would be very, very bad. If we were to scale fines by income then the middle or upper middle should be the point where there's not much difference (i.e. those people pay the same under the current system and some hypothetical income based system) because that's the point you need to be at for fines to be roughly proportional to the offense. I have zero faith that that could be implemented. What would likely happen is that the poor would still get screwed and everyone else would get just as screwed which is worse than the current situation.