Fermentation is a crucial step in idli-making as it helps the batter rise and develop a fluffy texture. It's possible that the batter didn't ferment properly or for long enough. Factors such as temperature and humidity can affect the fermentation process. Ideally, the batter should be left to ferment in a warm place for at least 8-12 hours, or until it has increased in volume and developed a slightly sour smell.
The ratio of idli rava (coarsely ground rice) to black gram (urad dal) could also impact the outcome. The typical ratio for idli batter is 4:1 (four parts rice to one part dal).
Although the image you shared clearly represents that the batter was not properly fermented. The same thing happened to me and I came to
conclusion that my proportion was not right which led to non-fermentation.
Hope this helps.