Setting the Popular race trait to -1 or -2 currently sets the effect to 0 rather than actually reducing income. This has been changed to fix the issue with negative income from tourism. As a result, Popular has now turned into a dump stat. I know 3.7 is supposed to address this, but I thought I might post my finding anyway - if nothing else it can help people who for whatever reason can not or will not upgrade to 3.7:
I have a suggestion which will negatively affect tourism income while stilll avoiding negative income: change the effect of this race trait from FLAT to MULTIPLIER, using negative multiplier for the settings unpopular (-1 or -2).
As an example, here is what my current definition of Unpopular(-1) looks like:
And here is the effect in game, on a planet with +2% Tourism from buildings:
As you can see, the -0.15 negative multiplier reduces the effective tourism income from 2% to 1.7%.
For the positive modifiers, I suggest to use both a flat and multiplier modifier, say 0.01 flat, and about 0.1 to 0.2 multiplier. The flat part ensures there's at least a modest amount of tourism to apply the multiplier on. Here's a screenshot of the tooltip you will see in Civ Builder:
The two percentile numbers can be a little confusing: the first number adds to the percentage of income you get from your controlled parsecs, the second is a multiplier applied to the total percentage.