IIRC, the ACLU is challenging a federal law which *requires* the use of filters for libraries which receive federal support. If you believe that the decision should be made at the local level, then you actually agree with the ACLU. (Don't worry, I won't tell anyone).
By its own stated policy - ACLU Policy 4 - the ACLU opposes on First Amendment grounds (1) "Laws which punish the distribution or exposure of such material to minors" i.e., obscenity, pornography or indecency and (2) "laws that restrict the production and distribution of any printed and visual materials even when some of the producers of those materials are punishable under criminal law," i.e, child pornography. The ACLU does not accept any form of filtering because it believes it's unconstitutional to prevent the distribution of pornography to anyone, including children.
The ACLU opposes filters in all cases.
I think you know the ACLU wouldn't care if it was done at the city, county, or state level.