Canon
The official Star Wars Databank has this to say:
If ever one needed an example of the irredeemable evil that was the Empire, turn to the shattered remains of Alderaan. An influential world, Alderaan was represented in the waning days of the Republic by such venerated politicians as <PERSON> and <PERSON>. A peaceful world, Alderaan was bereft of weaponry in an era of galactic strife. It was not without spirit, however. Alderaan was one of the earliest supporters of the Alliance to Restore the Republic, though its officials prudently kept all ties to the Rebellion secret. 503
Despite such discretion, the Empire knew it to be a haven of Rebel activity, and Grand Moff <PERSON> targeted the beautiful world for reprisal as soon as the Death Star was operational. The massive primary weapon of the battle station obliterated Alderaan, leaving only a lifeless asteroid field behind.
Wookieepedia (citing Star Wars Rebels – "A Princess on Lothal") says this:
During this time, Alderaan became the Alliance's main source of munitions. The planet's crown princess and representative in the Imperial Senate, Princess <PERSON>, adoptive daughter of <PERSON> and <PERSON>, began using her diplomatic immunity as an Imperial senator to carry out Rebel missions in restricted Imperial systems.
Legends
In Star Wars: Battlefront II, a 501st trooper says this:
"For all their talk of being a peaceful planet, Alderaan had been thumbing its nose at the Empire for years."
The Star Wars: Imperial Handbook: A Commander's Guide labels Alderaan as one of the Empire's priority targets for providing political and strategic aid to the Alliance.
Wookieepedia says this (although they didn't cite their source so take it with a grain of salt):
Immediately after the formation of the Galactic Empire, Alderaan was wracked by anti-Imperial protests, mainly from alien refugees who were now forced to pay an exorbitant tax to return home. Alderaan eventually became a safe haven for rebellious elements who wished to fight the growing oppression of the Empire, which helped bring on the planet's very downfall.
Included among the population was a group of Caamasi refugees depicted in the novel I, Jedi, for example, that fled to Alderaan after an Imperial bombardment destroyed their planet. They were most definitely Rebel sympathizers.
Conclusion - YES
While it's hard to determine what percentage of the Alderaan people were pro-Rebellion, it had a reputation for being very sympathetic to the Alliance which should be sufficient for the question.
It's generally understood that a significant portion of the galaxy's population didn't like the Empire but any official stance against the regime would carry significant consequences and violent suppression. Therefore, Alderaan, being a peaceful planet, was officially aligned with the Empire but both its political figures and its people had a reputation of secretly being very influential supporters of the Rebellion. 503