While the Wikipedia article on the Third Servile War has a few details on the effects of the slave revolt (e.g. less harsh treatment, reduction of the number of slaves on the latifundia), it says nothing about what Romans, prominent or otherwise, thought about the crucifixion of 6,000 slaves along the Appian Way. Nor is it totally clear (after checking several sources) who authorized the mass crucifixion, or whether Crassus actually needed anyone's authorization.
Although I do not doubt that many Romans approved of his actions in crucifying and then leaving them to hang on the crosses for several months, Crassus was more ruthless (and unscrupulus) than most so can we just assume that no one voiced concerns about the gruesome spectacle along the Appian Way?
With reference to sempaiscuba's comment on why slave-owners would care, some slave-owners' children were brought up with slave children. Thus, some slaves
were the confidantes and even friends of their masters and might receive educations, have their own families, and live nearly as well as the free members of the family
Source: Gregory S. Aldrete, 'Daily Life in the Roman City'
One potential source of dissent or disagreement might be stoics. Although most stoics did not oppose slavery, there were clearly concerns about the brutal treatment of slaves prior to the Third Servile War.
Stoics certainly offered a view of slavery that was more liberal than the contemporary views of the rest of society, and were united by the belief that masters should treat their slaves in a more humane way than they seem to have generally been treated. In Diodorus Siculus' account of the Sicilian slave-rising of 135-132 BC, much of what he wrote was heavily drawn from the Stoic philosopher Posidonius. Crucially, the depraved natures of the rich slave-owners such as Damophilus are blamed for the behaviour of the slaves rather than the slave's innate savagery.
1. Who authorized the mass crucifixion, or did Crassus not need anyone's authorization?
2. Were there any dissenting voices in the senate or among other prominent Romans concerning Crassus' actions? There were a number of senators in the late republic who were not afraid to speak their minds (at least on matters other than slavery), even at the risk of their lives (e.g. Cato the Younger, Cicero).
3. Is there any evidence of Romans complaining about the corpses lining the Appian Way? Presumably, at least some Roman travellers, including prominent ones, would have found the scenery rather displeasing to the eye. Is there any evidence to be found in, for example, poetry or plays?
Note. Other sources checked include:
The Cambridge Ancient History