Wouldn't sneak attacks conflict with their notion of honour?

That depends. The definition of honor is not universal in Star Trek. Hell, it's not even universal on Earth. According to bushido, a Japanese moral or 'honor' code, if you sneak attack and your opponent is unprepared then that's your opponents fault. Not yours. In the Star Trek Universe, Klingons see things very differently than humans when it comes to honor. A good example would be the TNG episode Ethics where Worf insists on an honorable death despite this horrifying Crusher. Worf also said in DS9 that "In war, nothing is more honorable than victory."

That being said, the Klingons are INCREDIBLY hypocritical when it comes to honor even by their own standards. For examples, look at literally anything to do with Worf and the Klingon High Council. Ezri Dax said "I see a society that is in deep denial about itself. We're talking about a warrior culture that prides itself on maintaining centuries-old traditions of honor and integrity. But in reality, it's willing to accept corruption at the highest levels."

So the Klingons being okay with cloaking boils down to one of two reasons, at least for me. The first option is that they're in denial about the fact that it's dishonorable. The other is that their alien way of looking at Honor just happens to cover cloaking as something that's okay.