This is exactly the reasoning behind having a "red paint job" upgrade for their vehicles. They think it will go faster, so that makes it go faster.
The Tau ARE actually grimdark. Their leaders mind control the lower classes. They lobotomize races to make them fit better into their society. They convert everyone by the sword to their greater good. Eldar have known they are just waiting for extinction for 10,000 years. They force asceticism on their entire culture to try to survive a little bit longer. What's not grimdark about that? I'm not arguing about looks, so what the Wulfen look like doesn't matter.
The issue isn't the spelling, it's that literally every single thing about the army gets its name from the same bad joke. If even half their army had actual names (instead of just misspelling what a child would call the unit), I would feel a lot differently.
This is an irrelevant point. No one is saying Orks don't like war, or aren't doing well in the fluff. What I am saying is that, for a setting that has gone so far out of its way to seem hopeless and doomed that a new word had to be invented to describe it, having a whole race for comic relief is annoying.
We're arguing over taste, so we're never going to agree.
The red paint job is one thing in the whole ork fluff that "works cause they want it too" there's no mention of any other thing that works like this it in the new codex at all. Have you read the new or codex? It's much less "goofy" than 2nd edition. In fact Orks havent been "goofy" for quite a while. However, as an ork player, I like the fact that they are not too serious, as to me this is a game, and it should be fun and entertaining. My job, mortgage and degree are serious enough, I don't need to take playing 40k seriously too.
If Orks weren't part of 40k, it would be a much changed beast, a lot of people would never have played it, and I think it probably would never have got off the ground. Orks are the ultimate protagonists for the imperium. The imperium is dour, and fretful, and violent and cruel, Orks are all also violent and cruel, but they're also cheerful. They're the only race that is. They are the antithesis of the imperium, which takes itself far to seriously. That's appealing to me.
Tau and eldar may have their faults, but they are not grimdork, that is solely the preserve of the imperium. All the other races have their own agendas and care not a whit for high gothic or nuns and monks in space, (and wacky space vikings).
Ork spelling is part of their kulture. It's not childish because they don't have children, it's simplified gothic, which as I mentioned before is impressive in its way. A brutal race with the physiology of a 400 pound gorilla that can speak a language with a jaw the size of a dustbin lid designed for squishy pink monkeys is quite a feat. The spelling reflects the simplicity and brutal mindset of Orks, if it was all Death Blitz fighter and the like, then it would be odd, because it doesn't fit their society. I've never heard of someone rubbishing an entire faction because they don't like the names.