"Great" smells very subjective. I went to https://forum.nim-lang.org/ . Put "flask" in the search box. Second hit (https://forum.nim-lang.org/search?q=flask) is this: https://forum.nim-lang.org/t/11032 . That mentions not one but 2 projects (https://github.com/HapticX/happyx & https://github.com/planety/prologue).

If either/both are not "great enough" in some particulars you want, why not raise a github issue? (Or even better look into adding said particulars yourself? This is really the main way Python grew its ecosystem.)