If you drive in the UK, you'll see road signs that have one word on them - STOP - and this is perfectly acceptable. English learners are sometimes taught as a "rule" that a simple sentence must have a subject and a verb as a minimum, but that isn't the case with orders.
A sign, a slogan, a newspaper headline - these don't have to obey the rules of grammar. "Stop Asian Hate" is punchy and succinct - it seems clear to me that it is a campaign to stop hate against Asian people - what else could it mean? The only other alternative would be that it is a campaign to stop Asian people hating something else - but what? It wouldn't be a very good slogan if there was no object.