Sorbet tastes less sweet when frozen?

Cold temperatures make you perceive flavors less in general. You can notice this quite directly when well-frozen eating sorbet or ice cream: you'll taste the flavor much more strongly initially than later on. And it's not just you getting used to the flavor. If you switch to something else, you'll still have a dulled perception of its flavor. I believe this is all primarily because your tongue and palate are numbed a bit. Your sense of touch doesn't work as well if your hands are freezing either! The lower temperature also reduces volatility, so aromatic things won't be as readily available to perceive.

As for sweetness, there's not exactly a rule of thumb per se for sorbet. I guess it's common to be somewhere in the ballpark of 1/2 to 2/3 a cup of sugar for 2-3 cups of fruit, but that's often more about adding enough sugar to keep it soft than anything else. In many cases, you end up adding lemon juice or something else to cut the sweetness back down, or perhaps using less sugar but adding alcohol to soften it.

In any case, you can certainly develop a sense of how sweet your sorbet will be before freezing, but it's hard to really convey in words. It's just a lot sweeter than you want it to taste in the end. Tasting a known good recipe might help train you more quickly, or perhaps tasting melted sorbet - but in that case, if you've just eaten a bunch of it, your perception is probably a bit skewed.