I see Nikiya as being happy twice: in her PPD with Solor, at least in part, and where she receives the basket. If she doesn't show that she thinks it's from Solor, then there's no way of making sense of why she embraces it, as opposed to it being a dance prop, and it's the only way for us to see what it represents: her assumption that Solor will remain true to her, even through she's just learned he's engaged to Gamzatti and has been behaving like an engaged man, if publicly, like Radames in "Aida." She dies before we see whether he'd have put his money where his mouth is, like Radames, or whether he's a weasel, like Albrecht, at least Act I Albrecht.
The whole business with the happy face during the basquet dance is a little tricky. She can certainly try a "defeated" sad smile as if telling Gamzatti..."You've got the Prince and this is my consolation prize..?, So be it", but certainly one happy wide grimace is just too much. Solor is right there next to his fiancee...a now forbidden territory for her, so it does not make sense to portray her sooo happy I think.