Resisting attacks that lower max HP...

[Mechanize]

If I resist damage to a necrotic attack that lowers my max HP does it half the max HP reduced as well? or is it essentially pointless?

[Valmark]

Assuming it's the same wording of something like Harm then yes, resistance halves the max hp reduction too.

It says "by the damage taken" and obviously if you halve the damage you receive you'll halve that too.