Something that is excellent is remarkably good. The synonym sterling means “thoroughly excellent,” but its history adds a little shine. Sterling entered English as a noun referring to the silver penny of the Norman dynasty. Over time, it came to be used to refer to British money more generally, as in "pound sterling." Along the way, sterling developed several attributive senses relating to money and more specifically, to the quality of metal used in money. This is where we get the phrase “sterling silver,” designating silver that has a fineness of 0.925, now used especially in the manufacture of table utensils and jewelry. The use of sterling as an indicator of quality was extended to things immaterial, such as a sterling reputation—to indicate that they meet or exceed the highest standard.
To lessen something is to make it less, or to reduce it. If you hope to lessen the chances of something bad happening, you want to make the number of chances fewer. Mitigate is a strong synonym for lessen, but this verb is used especially to talk about making things more bearable or giving relief, as to someone in pain or sorrow. It’s also used to talk about making things less severe: to mitigate the effects of climate change. Mitigate is sometimes confused with the similar sounding verb militate, which means “to have effect or influence.” This mix-up often occurs in the use of the phrase mitigate against, e.g., This criticism in no way mitigates (read militates) against your going ahead with your research. Although this use of mitigate occasionally occurs in edited writing, it is rare and is widely regarded as an error.
Something that’s so-so is neither very good nor very bad. If you go to a concert and report back that it was so-so, it means that you neither loved nor loathed the show. Rather, it was okay. Mediocre is another way to describe things that are middling or average. Though keep in mind, that describing something as average is, generally speaking, not exactly a glowing review! While this adjective appears on paper to be neutral, in most applications, mediocre suggests at best mere adequacy, as in a car that gets mediocre mileage, or at worst inferiority or poor quality: Mediocre construction makes that building dangerous.