For a mechanic to be "diegetic" it is required that all input to the mechanics is diegetic (in-fiction elements, knowable by people in the fiction) + a randomiser.
Player or DM preferences are not allowed to enter it either... if a DM let their preferences affect the mechanics, the result is no longer "diegetic".
Here is the text for RQ Dance skill
Dance (10)
This skill measures the adventurer’s ability to perform a dance—be it a social, ceremonial, erotic, martial, or sacred dance. The skill includes the knowledge of the different forms of dance known in the wider region (comparable to the Homelands described in this book).
Dance may be used to evoke an emotional or magical response from its audience. For example, a dance may be used to inspire lust, wonder, or even to tell a story. Dance may be used to augment Magic skills or the chance of someone casting a spell. Finally, Dance may be augmented with the Sing skill (or vice versa) to increase the chance of success of either skill.
A successful Dance skill evokes the desired emotional response from its audience. On a special success, the performance is extraordinary and accomplishes exactly what the adventurer intended. On a critical success, the dance evokes an intense emotional response from its audience: awe, lust, sadness, wonder, or whatever else the adventurer intends. The dancer gains a +25% bonus on all Communication rolls with the intended audience for a season.
Dance is in the Communication Skills category meaning that it is modified by INT, POW and CHA. A character who desired to could start with around 40% in Dance. Communication Skills have the following general rule
An adventurer uses such skills to transmit by word or gesture their thoughts and desires to others. Good communication skills usually leave a good impression.
Is Dance "simulationistic" according to your account? What are players and GM permitted to say as a result of an ordinary success with Dance? What about with an ordinary failure?
Furthermore for a mechanic to be "diegetic" it is required that all possible outcomes from the mechanics must be possible to understand trough a diegetic causality perspective from the inputs to the outputs.
However, the output do not need to contain any more information than a simple binary "yes/no".
It is possible in RQ to "Fumble" a roll to Dance.
Fumble
If an adventurer can succeed spectacularly, they also can fail spectacularly. Such a failure is called a fumble. The possibility of fumbling extends beyond combat and applies to every ability roll made in the game.
The chance of a fumble is equal to 5% of the adventurer’s chance of failure, and always occurs on a roll of “00”. As with the critical success and the special success, the chance for a fumble depends on the modified percentage roll for the situation, not on the adventurer’s normal chance with the ability.
A fumble is the worst possible failure and usually has disastrous consequences. In an opposed roll, a fumble is always worse than a failure.
When a success falls under multiple categories, always use the best result. Similarly, a failure should use the worst result.
What is player or GM permitted to narrate for Dance on a Fumble? What are the possible outcomes?