I assume you mean Aurebesh. The alphabet itself is canon, having first appeared fairly prominently in Return of the Jedi, and getting specialeditionized into the other ones. I guess your question though is, are the values assigned to each character canon or not.
I looked at the standard values given for Aurebesh in images on the internet, and then compared them to a screen readout from the beginning of Return of the Jedi
. The values returned nonsense (e.g. "meonghm yqxaej eonghmn"), and since I am sure they did not create a whole constructed written language for a couple computer screens when the spoken language clearly is English, it does not appear that the standard values of Aurebesh apply to Return of the Jedi
. However, the image you reference, Poe Dameron's vest, does in fact say "pull to inflate" in Aurebesh using the circulated readings, and even the special edition of A New Hope uses Aurebesh as a cipher of English
. So, since it appeared in more than one canon Star Wars film, it appears that yes, it is canon.
Also, several of the letters in RotJ differ in appearance from the ones that appear in other films. I'm almost certain this is what happened:
When they started making the special editions, they decided that they wanted to replace all English labels with the alphabet from RotJ. However, a decade had passed since that film was made, and most likely they could find no records of what the letters of the original alphabet actually stood for. Wanting to preserve visual consistency but not caring for coherence of some random symbols, someone probably put on RotJ and paused it in the parts with the alphabet, wrote down all the symbols they saw by hand (accounting for the distortion of several of the letters), and then randomly assigned Latin alphabet values to them, which are consistent with the internet images and their appearances in the actual Star Wars
What might be fun is to look at the original RotJ images and try to decipher what the original values of the letters were, if they actually had any to begin with.
EDIT: I was slightly wrong, and my thing seems to have been answered:
Wookieepedia wrote:An Aurebesh-like script first appeared in the 1983 movie Star Wars: Episode VI Return of the Jedi, the last installment in the original trilogy of Star Wars. It could be seen on monitor readouts on the second Death Star at the beginning of the movie, when Darth Vader's shuttle is scanned while approaching the battle station. Erik Schroeder's decoding of the technical readouts further suggest that this readout is illegible, consisting of lines of character repeats. However, it was Stephen Crane of West End Games who gave each character a name and a corresponding Roman letter or letter combination. At the time, West End Games's flagship product was the Star Wars Roleplaying Game. While he was writing the Star Wars Miniatures Battles Companion in 1993, Crane decided to develop an alphabet for gamers to use. Upon receiving Lucasfilm's approval, Crane came up with the "Aurebesh," a 34-letter alphabet. It was later expanded to include punctuation marks in Imperial Entanglements, a 1996 supplement to Miniatures Battles.
Stephen Crane's alphabet was subsequently adopted in many Star Wars works, and even made its way into the movies.
Anyway, I suspect Stephen Crane did what I said, pausing the movie and writing down by hand what he saw in order to create the new version of the alphabet.