No, the Aladdin sequels aren't canon. The only sequel that is a part of the Disney canon is The Rescuers Down Under.
Return of Jafar IS canon, actually. Aside from the fact that it actually gets rereleases, Iago was also highly marketed as being the first Disney villain to actually reform, which only happened in Return of Jafar (the fact that it even got a level in Kingdom Hearts II should be a huge indicator of canonicity as well).
The only films I've ever seen referred to as "canon" are the Walt Disney Animation Studios' feature films, of which there are currently 54 and The Return of Jafar is not one of them.
Quixotism wrote: The only films I've ever seen referred to as "canon" are the Walt Disney Animation Studios' feature films, of which there are currently 54 and The Return of Jafar is not one of them.
And if they weren't canon, don't you think they would have done to ROJ and pretty much any other sequel what Kojima Productions did to the likes of Metal Gear: Ghost Babel (ie, NOT rerelease them, certainly not give them special editions or boxed sets up to 2013)? And please don't try to claim they want money. Ghost Babel would have been a good cash cow, yet they aren't willing to rerelease it even on virtual console despite it being in high demand due to its noncanonicity, and besides which, Disney's already proven themselves to bury films even when it means losing a profit if they fear bad things happening, like what happened to Song of the South (buried because they fear being charged with being racists despite it actually being fairly anti-racist and if anything doing far more to actually promote the black community than what Al Sharpton and Jesse Jackson ever did) or Path to 9/11 (where they not only buried the film under pressure from the Clintons regarding a DVD release, they even went as far as to not even sell the rights to someone else who WOULD release the DVD if they won't). And if they could do that, they most certainly would have buried the DTV sequels to never see the light of day again (especially when those sequels would have been a painful reminder of the Eisner years).
Sequels I've taken as canon:
Other ones I either haven't seen or don't care about
Unless there's plot holes, it's all a matter of headcanon. If you want to think it didn't take place in the canon of the original film, nothing's stopping you.
As indicated by The Return of Jafar on this list, I do take it as canon, besides I'd rather think of Jafar as dead and Iago reformed. Aladdin is also the only Disney film in which I take its TV series as canon. (Hercules's TV series in particular has a gaping plot hole, ironic because it sees the temporary resurrection of Jafar)