Not all manipulation is created equal. Simon Sinek talks about manipulation tactics in Start With Why. Some manipulation is relatively harmless. I.e., a sale is a form of manipulation. Saying, “XYZ product is now 15% off,” is an attempt to manipulate someone to purchase said product. That doesn’t necessarily mean the person buying is getting ripped off. Likewise, Sinek’s larger point is that while manipulation may often be innocuous, it definitely isn’t the best method of marketing.
So, to answer your question, no, marketing isn’t slimy. But it can quickly become that way when you’re unclear on the onus for your product.