In some circles of the legal and business community, TV advertisements by lawyers are viewed as tawdry, and a sign of desperation.
The other answers adequately cover the fact that Jimmy ran the ad before having it approved, going against direct orders from Main. I will add to this that television advertising by lawyers is often viewed as a cheap, tacky move employed by unscrupulous types of lawyers seeking to hustle people into filing law suits. A form of televised ambulance-chasing if you will.
You may recall earlier in the series, Charles McGill actually bemoans the court decision that first allowed lawyers to advertise. In the legal profession, you want to be seen as not needing such "tricks," and relying on an excellent reputation to bring in business. Charles found ads like that manipulative and unbecoming a lawyer.
Cliff Main emphasizes that this perception goes beyond just what lawyers thinks. He tells Jimmy that such an ad could make D&M's other clients question whether they want to associate with a firm that runs ads like that, and that it would turn off prospective clients as well. People in the business community--the kind of clients D&M courts--want a firm with class and restraint, not a bunch of "ambulance chasers." It makes a firm look like it is desperate for business, which then begs some questions: Why are they desperate for business? What is wrong with them?
Main himself felt like a bit of a risk-taker, running his bland, text and facts-only TV ad for the asbestos claims (and only on late night TV). But such an ad that makes no appeal to emotion and just states facts as blandly as possible was the furthest extent that he felt a reputable law firm should go. Carting out an old lady and dramatizing her plight in such a manipulative way is simply too sleazy for high end lawyers.