Yes, that is Jim Carrey singing "Cuban Pete". (That is not, however, Cameron Diaz singing in the nightclub, as anyone who has seen My Best Friend's Wedding can attest.) Both Carrey and Diaz did all of their own dancing, Jim even helped choreograph the "Cuban Pete" number.
He also lives up to his nickname of "Rubber Face" when Stanley first becomes The Mask. CGI was used to distort the mask on his face, but the wacky, cartoonish faces? All Jim Carrey.
The Vikings burying the chest containing the mask on an American beach.
Peggy's death after betraying Stanley, where Dorian (who has the Mask on) hurls Peggy into a printing press, before a series of red-stained newspapers are printed with a fake story reporting her death. Because of that, Peggy disappears from the plot after getting her money and didn't get what she deserved. It was removed because of fears it would be too dark for the film, and an unrealized plan to have her appear in a sequel that never came to fruition.
An extended scene concerning The Mask's confrontation with a street gang.
An early workprint of the movie can be found online which has more extended/deleted scenes not available on the DVD. One of which is a scene that shows Stanley getting dropped off by his apartment after finding The Mask in the Edge City river, and is immediately confronted by the same street gang that he would see again shortly as The Mask. The gang's leader asks Stanley for the time so he can steal his watch, foreshadowing how Stanley would soon get payback on him as The Mask when asked the same question.
With that in mind, remember this exchange?
Mrs. Peenman: Ipkiss! Do you have any idea what time it is?!
Ipkiss: Actually, no.
The workprint also makes Tina come across as much more cynical compared to the final cut, as added lines make it clear that she hates the fact she's been forced into recording the bank's security in the opening scene, but does so because I Did What I Had to Do (and tells Stanley this in Landfill Park). This would have made her later speech to Stanley at the jail have more dramatic weight behind it, and it would have been more obvious that she had a more romantic view of life by the ending.
Dorian: You know what happened to the last bitch that ran out on me? Do you?! Tina: No... Dorian: Nobody else does, either. And nobody ever will.
Doing It for the Art: Carrey went to the extra trouble of practicing how to talk/act while wearing the Mask's enormous fake teeth, to enhance the comedic effect. Production had at first assumed that these scenes would be filmed with no sound from the Mask and dubbed later.
In general, the filmmakers ended saving a lot of money in special effects because of Carrey's talent for cartoonish gesticulation.
Dueling Dubs: In Spain, the film received two dubs: one for theatrical releases with Luis Posada as the voice of Jim Carrey and another for airplane screenings (yes, that is a practise although not very well known) with Antonio García Moral as Carrey. Thing is that when the film had its first Spanish DVD release, for unknown reasons they used the airplane screening dub for the Spanish audio track which was met with negative feedback. By the time, the film got a Blu-ray release and the distributor used the theatrical dub keeping the airplane dub for the DVD release. Curiously enough, both dubs feature Dionisio Macías in very different roles: Kellaway in the theatrical dub and Dorian in the airplane screening dub.
Executive Meddling: Very surprisingly inverted, at least if one listens to the DVD commentaries/behind the scenes material. Chuck Russell — director of A Nightmare on Elm Street 3: Dream Warriors— stated that New Line was interested in developing The Mask along the lines of the comic books; this would have become, according to Russell, "the next Freddy Krueger". But, he noted, that draft after draft of the more comics-faithful adaptation simply did not work. So, he fought valiantly to make the film as a comedy; the Cuban Pete sequence is one of the few elements that survived until the final cut.
There were plans for an actual sequel (notSon of the Mask, mind you). One idea was that Lt. Kellaway finds the mask, dons it, and becomes the next villain. Furthermore, Nintendo Power actually had a contest that whoever won could appear in the film as an extra. They Lampshaded this in their final issue.
NP: To whoever won that contest: sorry.
At one point, the studio worked with the idea of making the sequel about a woman (possibly Peggy) wearing the Mask.
Actor Allusion: In Bub's second appearance in the series, he at one point calls The Mask a "Neanderthal Ninny", which was one of the many insults Dr. Smith would use on Lost in Space.
Skillit looks a bit like a darker, more twisted version of Peter Pan from Peter Pan & the Pirates. (Heck, they even have similar powers, and The Mask at one point told Skillit to "Tell it to Tinkerbell".) This was most likely intentional, since they're both voiced by Jason Marsden.
Cowboy BeBop at His Computer: The season two premiere episode, "A Comedy of Eras" has been listed on some TV episode websites as being about The Mask meeting comic actors Adam Sandler, Mike Myers, and Jim Carrey. It's actually about The Mask going against a mad female scientist named Chronos who can manipulate time.
Expy: Doyle (Lt. Kellaway's partner) looks like a cleaned-up, less pathetic (but still kind of dim) version of Bill Dauterive from King of the Hill
Although since this cartoon predates that series, wouldn't that make Bill the expy of Doyle?
Both shows were also animated by Film Roman, presumably someone working on KOTH saw the Doyle model and used it as the basis for Bill.
Keep Circulating the Tapes: There are only three VHS releases (back when VCRs were popular) and the two-part episode "The Mask is Always Greener on the Other Side" was released on DVD along with The Mask around the time that Son of the Mask was released in theaters. Other than that, the entire series hasn't been released officially on DVD (it was announced that the show would see a DVD release in 2013 thanks to Shout! Factory but as of 2016 the set never materialized with no word as to why). All is not lost though — most of the episodes are available via torrent downloading and on video websites like YouTube and Dailymotion and the show did rerun on Cartoon Network and Boomerang channels in the UK and Australia.
On April 10, 2018, Warner Archive released the first season on DVD, though no further seasons have been announced as of 2019.
Recycled: The Series: A cartoon adaptation of a PG-13 Jim Carrey movie (two similar series that fit this description include "Ace Ventura: The Animated Series" - with which this show had a Crossover - and the short-lived cartoon adaptation of "Dumb And Dumber")
In the French dub, Emmanuel Curtil (Jim Carrey's official French dub voice actor) voices Stanley Ipkiss/the Mask, reprising his role from the movie.
In the English version, Ben Stein reprised his role as Dr. Neuman, the only actor from the movie to do so.
The European Spanish dub had all the voice actors reprising their roles from the film including Luis Posada as Stanley Ipkiss / The Mask. The sole exeption was Doyle who was voiced by Joaquín Muñoz instead of Miguel Rey.
What Could Have Been: The writers had plans for an episode involving the deceased spirits of Dorian Tyrell and crime boss Niko to return to Edge City and engage in a mob war from beyond the grave, with further plans to have actor Peter Greene return to reprise his role as Dorian. The show's cancellation ultimately canned this idea.