BUT can`t a lottery winner also add a fraudulent block to the chain? What does this lottery contribute to the validity of the chain?
No. If the block is invalid, it's ignored by everyone.
The "lottery" only determines which chain of all fully valid blocks is the right one. For example, say I have 1 bitcoin and I sign two transactions, one sending that bitcoin to Alice and one sending that bitcoin to Bob. Both those transactions in the same chain would be invalid. But you can imagine two chains -- one with the transaction to Alice and one with the transaction to Bob. Both are valid, which is right. That's what the mining lottery is for. It solves the "double spend" problem without a central authority to arbitrate.
Or in the other way: How are miners approving a bitcoin transaction, besides adding it to the blockchain when they win? Potentially, they could also add a lot of "sh**" to the blockchain.
The same way everyone else does. If they add a transaction other people won't consider valid, then their block will be ignored and they will have wasted the effort they spent mining it.
Blocks that don't pass all the requirements for validity, including checking every transaction in them, are totally ignored. Honest clients will never relay them to other clients or consider them part of the valid block chain.
Every participating Bitcoin full node can verify every single transaction in every single block.