If you mean "a short, memorable phrase used in advertising or politics":
* Longwinded explanation: This contrasts with the brevity of a slogan.
* Detailed description: This emphasizes the lack of succinctness found in a slogan.
* Unmemorable statement: This focuses on the lack of impact and catchiness.
If you mean "a rallying cry or motto":
* Discouragement: This is the opposite of inspiring or motivating.
* Indifference: This implies a lack of enthusiasm or passion.
* Division: This conveys the opposite of a uniting call to action.
If you mean "a catchy phrase used to promote a brand or idea":
* Bland statement: This emphasizes the lack of originality and impact.
* Generic phrase: This highlights the lack of specific branding.
* Unremarkable message: This focuses on the lack of memorability and distinctiveness.
The best antonym will depend on the specific context and what you're trying to convey.