RFC 5952: A Recommendation for IPv6 Address Text Representation
says "The characters "a", "b", "c", "d", "e", and "f" in an IPv6 address MUST be represented in lowercase "[1]
One Reason for this, is that it can avoid redundancy if you submit a hexdecimal nummber by phone.
"However, when a letter is spelled uppercase,people tend to specify that it is uppercase, which is unnecessary information."[2]
If you care about typography and think mixing lower-case letters with upper-case numbers, you could argue, that oldstyle numbners aka. lower-case numbers could be a better fit.