There is a huge variety in styles - gymnasts wearing makeup, gymnasts without, gymnasts in unitards, gymnasts in leotards, gymnasts with bows, gymnasts with plain ponytails or buns.
The gymnasts are mostly adult women these days.
If you were watching yesterday and saw team USA or Brazil - they were very sparkly, mostly with lots of makeup. These women are superstars and dress like the celebrities they are, in line with their home cultures.
Canada, China, Romania, Italy, UK - mostly much more workmanlike, but makeup use varied by gymnast, as you'd hope. Germany - unitards. Japan - matching hair accessories to honour their captain who made them (long story).
Several gymnasts have businesses relating to sports accessories and fashion. Good for them.
Smiling is not required - on floor only, expressiveness matching the music is required. That's not necessarily smiling. Dance, movement and extension account for a small proportion of points on beam and floor. That's the nature of the sport.
I am bored by fashion and can't do makeup but these women are just behaving like women from a mix of cultures on the world stage. You are projecting here, based on the appearance of a small number.