Wouldn't that depend on her music and her routine. My littlest has "Winnie the Pooh", it is very cute and bouncy, like Tigger, and it makes everyone smile, so she smiles.
My oldest DD has a much different piece, "disneys Parade" and it doesn't lend itself to a smile. So, she tends to go for a benign happy face. Of course during tumbling that can look like the grinch, but hey, she's having fun.
I suppose in the US, with the level 4,5,6 compulsory music it is hard to make one perfect routine stand out from the other without the gymnast going that extra mile with a smile. (OOOH poetry) But that level 5 stripper music always makes me laugh.
I think some girls are more naturally smilers, a forced smile will look worse than no smile.

Just like that!!! She is probably best left alone and allowed to do it her way, when she gets to have her own music she may tend to choose the more dramatic pieces.