Why is DC offset from LO feedthrough less problematic in the second mixer stage of a dual conversion receiver?
The first mixer is fed by an LNA with a lot of gain making it more sensitive not only to the signal you're trying to receive, but also to signals you aren't trying to receive, including internal leakage. The dual conversion method uses IFs that are far enough away in frequency from the signal you're interested in that they are easy to filter out. 

Say you want to receive 100 MHz at baseband. In a direct conversion recover, you need a 100 MHz LO. If the signal our of your LO leaks into the antenna, it's indistinguishable from the signal you're interested in. However, a dual conversion system might use frequencies of 70 MHz and 30 MHz. If either of these leak into the antenna, it's trivial to filter out.

Basically, it's no so much about the mixer itself as what the mixer is connected to.