Generally speaking, blockchains provide the ability to immutably store data but in and of themselves they do not assure validity of that data -- that is the job of the process or protocol that adds the data to the blockchain.
As an example, if you are a university and you want to issue and store diplomas on a blockchain, you will achieve immutability -- the diploma will stick around forever, even if the university is shut down. The validity of the diploma may be based on trust -- "I trust Harvard not to lie that they issued someone a diploma, and I know Harvard created this record" -- or other methods like decentralized voting or attestation.
If you have a certificate that is immutable and digitally signed by the issuing institution, this is a pretty strong deterrent to impersonation and faking.