Star Wars is and has aways been a franchise with a sense of humor. Even the otherwise dark Rogue One has a sarcastic droid sidekick. I don't fault anyone for wanting to play the comic relief. The trick for the player is to only let their Jawa or Ewok get up to their antics when tonally appropriate. The Guardians of the Galaxy movies are actually good guidelines for this, I think. Groot and Rocket are both "joke characters" from an RPG standpoint, but they also deliver some of those movies biggest emotional moments.
As with anything, I'd err on the side of communicating with a problem player rather than banning "little" species. If their antics are getting out of hand, talk to them. Work with them. Learning to roleplay takes time.
Edited by SavageBob