I'm having the same issue with my ice cream restaurant. I also have popcorn and fruit salad as appetizers, and all the ice cream cones as desserts. But non played guests only ever order drinks. It's just so massively disappointing to see that this bug has been in the game for nearly 5 years.
My first time playing Dine Out and this issue is still happening on Aug 2021. I can only hope the devs will look into Dine Out during the upcoming pack refresh.
As crinrict already said, the only desserts Sims will autonomously order are the experimental dessert, but that is because those are tagged as lunch and dinner in the recipe_tags that can be found in the recipe tuning. Most other desserts, ice cream, baked goods and also most of the Mt. Komorebi meals aren't tagged as breakfast, lunch or dinner.
Sims will only order food that is classified as
<E>Recipe_Meal_Breakfast</E>
<E>Recipe_Meal_Lunch</E>
or/and
<E>Recipe_Meal_Dinner</E>
They will also only do so at the appropriate time of the day.
I don't know where it the settings for these orders are made but I have been testing this excessively and this is clearly the way it works.
Tagging does meals that way is not a perfect solution, though (except for the Mt. Komorebi meals, where the lack of tagging is actually a mere oversight).
The problem is that recipes that are tagged that way will also be considered appropriate in the situation where a Sim autonomously grabs leftovers. That will result in Sims grabbing ice cream all the time instead of a proper meal as soon as there is some ice cream in the fridge.
So the right solution would clearly be to allow dishes tagged as desserts to be ordered at restaurants.
