At times, I think she could find it in her to have fun like that! It actually makes a lot of sense, when you think about it. While she's unarguably got the most serious responsibility of any character in the Mario universe, as well as the most tragic past, considering how she went through puberty with Lumas (children), I believe she has retained much of the child-like spirit we've seen her display in her storybook. This is reflected in her life long hobby of having fantastical stories built up in her library.
While she may be divine and immortal, she began life as a normal, innocent child, and I'm sure the pressures she undergoes call for that stress to be released on occasion, which would include her acting like a big adorable dork like in the above pictures. :3
I believe even the most elegant and refined characters should always have at least one closet quirk that contradicts that, since it makes them more relate-able and gives them more agency, and prevents them from becoming too much of a Mary Sue. For example, Professor Layton is the most polite and eloquent of gentlemen, as well as being a genius - but he's also really messy, and has a tendency to end up sleeping in due to studying too much.
So yeah, I definitely think Rosalina has the capacity to be silly and adorable, on occasion! :D