The Korean war started in 1950. The B-36 was introduced in 1949. Older B-29 superfortresses were slaughtered by Mig-15 fighters. The B-36 was faster (700 km/h vs B-29s 574 km/h), flew higher (12,000 m vs 9,710 m), and could carry more bombs. Instead of deploying a B-36 aluminum overcast, the US only used a few of them for recon operations.
Why was that?
The B-36 was faster (700 km/h vs B-29s 574 km/h), flew higher (12,000 m vs 9,710 m), and could carry more bombs.
While it is true the B-36 flew higher, the B-36's cruise speed (200 knots) was barely faster than the B-29's (190 knots).
In addition, even with this improvement over the B-29, the B-36 still flew within the envelope of the MiG-15, although intercepting a B-36 flying that high would still have been a difficult feat for it.
Of course, if a B-36 is flying that high, it wouldn't make for a very effective bomber (and its payload would have been limited), which is another reason why the RB-36 recon version existed - it would have to fly high and stay high.
Finally, the B-36 was tasked with being the bomber which was to be the core of the immediate post-WW2 strategic nuclear force. It had far greater range and payload capacity than the B-29, and thus had operational necessity that drew it away from Korea. You see this during Vietnam too, in which the B-52G flew the majority of missions in Vietnam, despite the newer B-52H having already entered service. Most of those B-52H's served with SAC and their low hours led to them being the variant kept into service to this day.