Can future Bitcoin hashes be considered a good source to generate random numbers? There are a lot of miners trying to calculate the next valid SHA-256 hash using random strings, so can I consider that generated hashes have good randomness? How can I prove it?

For example, use the bitcoin hash as a entropy or seed to generate random numbers. How would you solve this problem? What algorithm would you use to generate random numbers with the bitcoin hash as a input seed? but the algorithm should be secure and have a good entropy
As Bonneau et al. showed a Bitcoin block header has high min-entropy, approximately 68 bits. So, in theory you could extract randomness for your protocol or application from Bitcoin block headers, but bear in mind that blockchain's entropy is malleable as shown by Pierrot et al.

For example, if you run a lottery whose randomness is extracted from the Bitcoin blockchain, a miner could interfere with this randomness by withholding blocks whose entropy is not profitable for her. She might want to do this, if she is participating in the lottery and the prize of the lottery is larger than the block reward.

To deter such an attack you might want to use verifiable delay functions(VDF). VDFs make impossible to learn the randomness derived from the block before some time-out (say 1 hour), since it requires sequential work to compute, therefore massive parallelization does not help here. Furthermore once the output of the VDF function is calculated it is easy to verify the computation given a small proof. By applying VDFs even the miner does not know the output of the VDF before the time-out. Therefore there would be no more financial incentive for the miner not to publish her block, since she would forfeit her block reward.