How did Captain (mirror) Georgiou reach Qo'nos without being noticed?

I am not aware of any canon explanation of Georgiou's arrival on Qo'nos, as no works seem to have covered this. However, her character was the epitome of "getting stuff done", which is why she was recruited by Section 31 in the first place. Her tactical genius, ability to think outside the box, and dubious connection to morality made her the perfect agent, and probably more than capable of smuggling herself to Qo'nos.

But, I think the real answer to the question is, that it just wasn't that hard to do anyway.

Getting into L'Rell's residence was probably much harder than just getting to the planet. Remember, the war was no longer going on, and many other races traded with the Klingons and even lived on Qo'nos, so either stowing away with them, flat out hiring them to smuggle her to the planet, or just stealing one of their ships is definitely something she could have done.

And, beyond all that, we do see in Saints of Imperfection that S31 have some form of cloaking technology. When Ash calls for Leland's backup to hold the Discovery in place while Stammets and Burnham enter the Mycelial Realm, Leland's ship does suddenly appear from nowhere. Memory Alpha (since I don't have time to re-watch that episode at the moment), claims it was holographically disguised as an asteroid, but we just don't know the extent of their capabilities.