No, the word ‘dementia’ is not a proper noun.
Proper nouns are specific names for particular people, places, or organizations and are always capitalized. Examples include ‘John’, ‘Paris’, or ‘Microsoft’. On the other hand, ‘dementia’ is a common noun that refers to a general category of cognitive disorders that affects memory, thinking, and social abilities to the extent that it interferes with daily functioning. Since it does not refer to a specific entity or title, it remains lowercase in standard usage.