I attach a patch for this bug.
It makes dired-mark-unmarked-files act case-insensitively. This seems reasonable, as there's nothing to suggest it does otherwise.
Also, it is better than Stefan's suggestion to use auto-mode-case-fold, as when visiting files, the user will quickly see if a file has been e.g. visited in the wrong major mode owing to an uppercase filename being treated as a lowercase one; whereas in dired-omit-mode, files can be hidden counter to the user's expectation, as happened to me with COPYING.LIB, yet it is not obvious (because the file is hidden!).
Is this OK to install?
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0001-Match-files-case-insensitively-in-dired-x.patch