I understand what you are trying to say, but impairment loss is not about price movement but it is about the ratio going off. By having a USD pair LP, you have higher risk of impairment loss.

For example, you put in 50% of token A and 50% of token B.
If token A double in value while token B remain the same(stable coin), you may suffer 25% or so impairment loss(even tho neither of the token fall in price).