Brion did work long ago {T4290: Disallow usernames that are too similar to existing names (confusables, impersonation)} to make sure usernames with different capitalisation wouldn't conflict. I've created users with an initial lower case letter and found that they work.
These users show up on an un-modified MediaWiki's RecentChanges page. You can search for the uploads on ListFiles. However some pages do not show these users.
For example, even though the user "lowercaseUser" had been created, going to [[User:lowercaseUser]], [[Special:UserRights/lowercaseUser]] and [[Special:Contributions/lowecaseUser]] would say the user doesn't exist.
Upon examination, all these cases relied on the value of the private MWNamespace::$ $alwaysCapitalizedNamespaces. Removing NS_USER from that list allowed the special pages to understand that the user existed and act accordingly. (Note, I also removed a couple of ucfirst() calls in User.php.)
Since T4290 ensures that users won't be able to create a user named "LowercaseUser", and "lowercaseUser" is only allowed normally if $wgCapitalLinks is false (not the default), I think this is a safe change.