In order to understand the quote from Henry David Thoreau, one must first define any questionable terms in the line.
For some, not understanding what "mutable" means could cause one to not understand the line or to misinterpret the quote.
Mutable refers to something which is able to be changed or capable of change.
The second part of the quote which needs explaining is the literal understanding of a cloud and the metaphorical understanding of a cloud.
First, a cloud is composed of either water droplets or ice crystals. Their composition, as a liquid or a solid, remain constant.
The way a cloud looks, though, is different. Clouds never look the same. First, there are many different types of clouds: Cumulonimbus, Cirrostratus, Cirrus, Cirrocumulus, Altostratus, Altocumulus, Stratocumulus, Cumulus, and Stratus. Outside of the many different clouds, clouds never look the same. Even with clouds being categorized within a specific group, they are not defined by how they "look" to the average person. Instead, clouds change before a person's eyes, never staying the same.
Therefore, this quote is definable in regards to how nature is like a cloud. Nature and clouds alike are constantly changing. While some look at a cloud, or nature for that fact, as something which is constant or uniform in make/manner, others look at nature (with appreciation) based upon the fact that it is never the same.
Thoreau, therefore, is trying to appeal to both the scientist and the romantic in all people. In the end, the quote is actually redundant and impossible based upon the fact that a changing cloud is both able to change and stay the same at the same time. Figuratively, the quote is a paradox.