Is is possible to read the built-in dictionary on iPad?
From what I've been able to figure out, the "dictionaries" in iOS are for word definition lookup within iOS and in iOS apps and aren't in book (human-readable) format. Unfortunately, none of the dictionaries are available for reading as you would read a book.
Apple Books has many dictionaries available. Here are two from the top of the list:
Webster's Unabridged Dictionary
Webster's American English Dictionary