So, why do the couple not want to eat the dessert? Is it because they know their suggestions were in fact terrible?
This is a three-panel drama.
The setup: Blondie and Dagwood are getting a free dessert from the chef.
The complication: Dagwood has made continuing suggestions in the past regarding the chef's shrimp scampi. The chef, like any professional, likely will resent the "advice" given by an outsider, especially if it is perceived as criticism of his product. Therefore, the free dessert is suspect and possibly dangerous.
The resolution: Blondie and Dagwood decline the gift, fearing it may be likely to do them harm.
In the US, it is widely thought that complaining to the waiter and/or chef could upset them, and result in them spitting in your food (or doing something else gross, like serving old or dirty food):
- Do Servers Actually Spit in Your Food?
- How commonly do employees spit in customers' dishes in restaurants?
- Cops Use DNA Analysis to Prove Chili's Waiter Spit in Customer's Drink
I doubt it actually happens all that often (and if it does, I'm not sure I'd want to know), but it is a widely known trope.
In your comic, Dagwood and Blondie are initially happy to receive free desserts, but after the waiter's (possibly sarcastic) comment about their "suggestions" (complaints) about the shrimp scampi, they can't be certain that the sundaes would be good/safe to eat.
When I read this one, I took it to be that Blondie and Dagwood are calling into question the quality of the sundaes, not that the kitchen staff is looking to take some sort of vengeful measure, but that they feel it will be of low quality, as they had to make suggestions to improve the chef's other recipe.
I don't think this is anything to do with the English language. The words are perfectly clear, however they don't make sense when put together like that.
The waiter's words are not obviously sarcastic.
Conceivably the couple made useful suggestions and the chef appreciated them. The "punch line" makes no sense to me. If the waiter had brought out a single pea on a plate, then it would make sense.
Why doesn't the couple want to eat the dessert in this comic strip?
There is nothing in the words to indicate that. It is a mystery. If you were highly paranoid, and thought that if you made suggestions for how to improve the meal, the chef would spit into your food and then offer you a free dessert, then maybe. But I think that is a long bow to draw.