Let's take a very shallow dive into what slogans are for and how to craft good slogans.
They're to convince people over to your side and provide something to rally around. This slogan fails both tests. The initial response people have is to think the movement is full of idiots. You can't rally around something when 1/2 your side hates explaining the damn slogan to people.
You craft good slogans by targeting people's motivations. A lot of people still equate police with safety. That slogan when read by those people reads "I want you less safe!" That's why it's a bad slogan. There are a shitload of stupid/lazy people in this world. They're not all malicious. Also, calling them malicious because they're susceptible to propaganda just makes the movement look worse. People don't like being insulted.
From above, better slogans:
"Save our cities!" People recognize there's something wrong. They want it fixed. This goes directly to that motivation. Pretty hard to attack.
"Stop wasteful spending!" People don't like wasteful spending. They want to pay as little taxes as possible. This goes directly to that motivation. Pretty hard to attack.
"Give our police the support they need!" Again, a lot of people equate police with safety. This goes directly to their safety motivation. Pretty hard to attack.
When your slogan gives people ammunition against you, you've already lost the messaging battle. You're no longer talking about reallocating funding to social services, you're trying to defend your slogan. Just look at this thread.;