Yes, it's in the plank but not explicitly
The Green Party's platform can be found online. You can view the July 2020 platform here. Though it doesn't mention "Citizen's United" explicitly, it does propose solutions that are inconsistent with Supreme Court's decision,
Such as public financing of elections in 1(a).
Provide full public financing of federal, state and local elections, including free and equal radio and television time on the public airwaves for all ballot-qualified candidates and parties.
And further, an explicit prohibition of corporate donations,
Prohibit corporations from spending to influence elections, preferably by constitutional amendment abolishing granting corporations constitutional rights guaranteed to natural persons, or as a condition of receipt of a corporate charter by federal chartering of corporations.
Note all of this is highly relevant because the conditional amendment is required to overturn the Citizen's United verdict which is a Supreme Court's decision.
It's also in their "four pillars",
We demand public financing of elections, open debates, and more representative voting systems.