Yes, you can cash out skins from other sites besides CSGO if you want. Many CSGO skins cashout websites also offer cashouts for other games, and you will be able to sell your skins for other games and get money in return. This way, you can get rid of skins that you do not need and only fill your inventory.
That being said, you will be able to cash out skins for games like Dota 2, Rust, and many others. You just need to find the right platform. While some websites only focus on CSGO skin cashout, others will offer services for both CSGO and other games simultaneously.