I gave my players a halberd which had a magical enchantment that's only purpose was to keep the weapon clean.
Would this be considered magical enough to overcome resistance against non-magical weapons?
That's entirely up to the DM. Do you consider the home brew weapon as magical for the purpose of bypassing resistance when the effect on it is so minor?
If the DM allows it, then yes, it is magical.
If the DM thinks it's just a flavour thing, then no, it is not magical.
There is nothing implied by the current rules to suggest that magical weapons do more damage than non magical weapons. For example, monk attacks eventually count as magical even though that doesn't give them a bonus to damage.
There is also nothing in the DMG that suggests that magical items with flavorful benefits don't count as magical weapons for damage resistance.
Therefore, RAW I would say such a weapon counts. However if the person who created the magical weapon feels differently, I would cede to their judgement.