Is there any Open Source Litecoin Wallet with built-in CoinJoin for both Mobile and Desktop (aside from Mustard Wallet)?
Why not use mustard? It is the wasabi implementation for litecoin.
I’d also recommend coinjoin your coins on a pc, then send your clean utxo to a separate mobile wallet to avoid future mixing of utxos
(Edit)
Nothing. I am just wondering if there is another Litecoin Wallet with CoinJoin Implementation. 
Ah, well did a quick search as well and came up empty handed.
Watch out for scam websites that are trying to steal your funds - for example:
Coinjoin io (first page result on google)
THIS IS A SCAM WEBSITE
Asks you to input your seed phrase online (NEVER DO THIS) with the excuse of them executing a coinjoin for you. So be careful out there everyone.
Regarding what I wrote about mobile wallet use above, do have a read at the Wasabi guide for this - linked in the Mustard wallet FAQ section: