Skins for CS:GO are very diverse, and not all experiments of developers and skinmakers are successful. It happens that the community considers some designs funny or ridiculous, and players do not want to see some skins in their inventory even for nothing. The only benefit from some of them – the ability to use in some exchange contract.
MP9 Slide

Perhaps the main problem with MP9 Slide – bad color combination. Orange and light brown on a light gray background don't look like much contrast and can look more like dirt than a thoughtful design. It's surprising that this design did not appear in the early years of the game's existence, but in 2018, when bright and detailed coloring received the most attention.
Nova Sand Dune

Perhaps Nova Sand Dune – one of the most primitive skins in the entire game. The entire body of the weapon is simply covered with a single-color sand-colored paint. The coating is decorated with only a few small details. Fans of minimalism may like this coloring, but most players will find it too boring.
FAMAS Colony

And FAMAS Colony. It's a khaki skin with dirt specks scattered across the surface. On the one hand, this gives it the realistic appearance of a weapon that has been in battle. On the other hand, the history of the skins market has repeatedly confirmed that players like bright, albeit unrealistic, skins much more, and the lack of interest in this option is proof of this.
Negev Army Sheen

This weapon itself is not very popular, but the skin Negev Army Sheen you don’t see very often. Its main drawback is that it does not differ too much from the standard weapon colors. For the very presence of the skin to be reliably noticeable, you need a version with at least the “Minimal Wear” status, since at higher floats the paint wears off so much that it barely becomes noticeable.
Navaha Knife Rust Coat

A skin for a knife depicting a surface covered with rust – also a questionable decision. Most often, players buy the Navaja Knife because it costs relatively little, but at the same time looks quite stylish. A layer of rust robs it of the second of these benefits. Although you can come up with excuses for this look, most players prefer other versions of this knife.
Which skins seem the most unsuccessful to you?