WebFinger uses the acct URI scheme [RFC 7565], which says "The userpart consists only of Unicode code points that conform to the PRECIS IdentifierClass specified in [RFC7564]", and refers to case folding/normalization ONLY wrt scheme, host and percent-encoding, not the username.
There is a UsernameCaseMapped profile for IdentifierClass [RFC 8265], but no ActivityPub related spec refers to it. Thanks to @aslakr for the references!