At least one Klingon has indeed said that killing without showing one’s face is dishonorable. It’s just that Klingons (the ones who rise to the top, anyway) go ahead and do it when it’s to their advantage.
There is at least one time when Klingons choose to attack without activating their cloaking devices. In the alternative timeline of “Yesterday’s Enterprise,” Commander Riker remarks on the audacity of the three Klingon battlecruisers not even bothering to cloak. Although we never see the Klingons’ perspective, the clear implication is that they’re confident they don’t need any more of an advantage. It might be that the Klingon commander believes cloaking when he or she outnumbers the enemy three-to-one would seem overly cautious, or that they considered it more honorable not to cloak if they didn’t need to. However, since the Klingons would have known that the Enterprise could outrun but not outgun them, they might have been trying to intimidate Picard into running away. Or it’s possible that one of their ships’ cloaking devices was broken.
Klingon attitudes to the cloaking device are almost entirely pragmatic. This is particularly clear in the TNG episode “A Matter of Honor,” where it’s a major plot point that a Klingon first officer is supposed to assassinate a captain who becomes weak or unfit, but it’s never even suggested that there’s anything dishonorable about cloaking to ambush the Enterprise, and the Starfleet crew simply assumes that’s what the Klingons are up to the moment they don’t see them.
Out of Universe: Klingons Weren’t Honorable Until Twenty Years Later
That aspect of Klingon culture was established on The Next Generation. It had developed in non-canonical works first (particularly the novels of John M. Ford, one of which the actors playing Klingons in the first season of Discovery were asked to read) and was later retconned into Star Trek canon set prior to TOS. However, Klingons weren’t depicted as being obsessed with their honor before around 1988. Even then, it was a targ-eat-targ galaxy. Klingons got cloaking devices on Star Trek long before they got an honor culture.
The original idea of the Romulans (who invented the cloaking device) sharing technology with the Klingons was just an excuse to re-use props and keep “The Enterprise Incident” under budget.
The first Klingons confirmed to have cloaking devices on-screen didn’t care about honor at all. The first Klingon ship with a cloaking device appeared in the animated episode “The Time Trap.” The Klingons there were treacherous liars who plotted to plant a bomb on the Enterprise while pretending to work together to escape the eponymous trap, and then as soon as that was foiled, they completely fabricated a story about how they alone saved the day.
You can see one minor Klingon character act with something resembling honor in the first canonical appearance of a cloaked Klingon ship, in Star Trek III. It’s a hint of what Klingons would act like from then on, alongside how they’d always acted before. The Klingon spy there understands when her lover kills her because she has seen the top-secret document she just gave him. The villainous Klingon captain who kills her tells her she “will be remembered with honor.” But he uses his cloak to ambush and destroy a freighter, then brags that now he won’t have to pay them like he promised.
In the Star Trek movies made while TNG was in production, the Klingons start talking more about honor, but they have no compunctions about firing while cloaked, framing someone else for an assassination, or claiming that Shakespeare wrote in Klingon.
In-Universe: They Were Never That Honorable
There’s a line in the TNG episode “Reunion” where the dying Klingon leader tells Picard, “The Klingon who kills without showing his face has no honor.” In context, however, he’s asking Picard to solve the mystery of which Klingon warlord has done just that, and expressing remorse about all the dishonorable things he has done for the good of the Empire, such as covering up the truth about Worf’s father. The rival who wins that succession crisis turns out to be not much better. In the same episode, the half-Klingon Ambassador K’ehleyr says cynically that a Klingon civil war is starting because of “The usual excuses. Tradition, duty, honor.” This theme keeps reappearing until Worf finds out that he himself will be assassinated after promising the House of Mogh will stop the cycle of vengeance, and be remembered as a weakling. Klingon honor is honored more often in the breach than the observance.
Even in the episode that established the Klingon warrior tradition and its rituals, “Heart of Glory,” the honorable, religious warriors are self-consciously fundamentalist separatists from a mainstream they see as decadent.