In medieval Europe a claim typically helped, however it wasn't vital and there are many instances of individuals gaining titles that they had no claim to, especially during the early middle ages. Towards the end of the medieval era many claims were based on being related to a previous holder of the title, and since the number of descendants increases exponentially with each generation, almost everyone was related to the holder of a title by the end of the medieval era.

There are various instances of fabricated claims during medieval times. These fabrications come under four main categories. Pretending to be someone you were not, Forging a historical document that said you have a claim to that title, Marrying someone with a better claim after(or while) you've taken the title, Lying (or revealing a truth) about how the current holder of the title doesn't have a claim to the title.

Pretending to be someone you weren't isn't something that exists within the mechanics of CK2, historic attempts at this include Perkin Warbeck who claimed to be Richard Duke of York in 1499. In 1300 a woman claimed to be Margaret Queen of Scots in Norway, despite the fact that she was about 30 years too old. There are no known instances of someone successfully lying about their identity (although if they were successful then obviously we wouldn't know about it)

Forging a historic document is the one that is most similar to the way fabrication happens in CK2 however it only rarely happened during the medieval era. The most famous example of forging a document is the Donation of Constantine, where during the 8th century the Catholic Church forged a document claiming that the emperor Constantine had given the Bishop of Rome and the Catholic Church authority over all other Bishops, as well as all the land in the western half of the Empire. This document was believed to be genuine until the 15th century, and is by far the most successful forgery of the medieval era.

Marrying someone with a claim isn't fabricating a claim as much as it is gaining a legitimate claim. Examples of this include Ivalyo of Bulgaria who had lead a peasant uprising in Bulgaria in 1278, after he had overthrown the previous monarch he married his wife Maria to gain a claim to the throne. Another example is William Marshall, he gained a claim to his title of the Earl of Pembroke by marrying the daughter of the previous Earl (typically titles weren't transferred this way in medieval England)

Instances of discrediting a previous holder of a title include claiming Richard III claiming that his brothers children were all bastards who had no claim to the throne because his brothers' marriage was not valid.

In summary fabrication of claims in the way it's presented in CK2 was not feasible in most instances, and were typically not necessary in order to gain a title in the first place. The most effective way to gain a claim during the medieval era was through marriage.