There are three kinds of evidence in play here:
1. Artifacts claimed to come from yeti. High probability of being fake, whether the people who possess them are aware of that or not.
2. Physical evidence collected in the wild. High probability of coming from a more common animal, like a bear, than something less common.
3. Sightings, whether directly by the reporter, or from a first-hand witness describing to a reporter. Impossible to prove, but there are a lot of reports. Rule out everything that can be mistaken identity or fraudulent, and there are still a lot of reports. There are many consistencies across these reports that can't be explained as simply common-knowledge about expected behavior and characteristics. That's not proof, but it lends credence to the possibility of a real creature.