If we are looking at the similarities to pronounce Memoji and Mimoji relating to trademark definition it is not going to hold up in court.
Trying to prove copyright of the avatars is going to be challenging as well, it may resemble but prove financial loss. It is a weak case and burning money on a lawsuit that there is little to no possibility of winning is asinine.
A
trademark is a word, phrase, symbol, and/or design that identifies and distinguishes the source of the goods of one party from those of others.
A
copyright protects original works of authorship including literary, dramatic, musical, and artistic works, such as poetry, novels, movies, songs, computer software, and architecture.
Source:
https://www.uspto.gov/trademarks-getting-started/trademark-basics/trademark-patent-or-copyright