Thumbelina is, and has always been, your typical fairy tale "heroine" whose entire existence consists if being tossed around by all available parties. When she's not being betrothed to frogs, she's being hit on by beetles. But of course, none of those will do at all, because they aren't tiny people like Thumbelina is. See, Thumbelina is a bit of a racist, and the whole theme of the story is how people of different races can't be together. It doesn't explain why, just that they can't. But that's neither here nor there, the story was written hundreds of years ago after all. All that really matters nowadays is how the story is adapted, and... it tends not to go well. As mentioned before, Thumbelina is never particularly active in her own story, constantly tossed around by all available parties like they're playing volley ball. While Belle stood up for herself and Rapunzel left her tower, Thumbelina remains a perpetual damsel.
Don Bluth's Thumbelina
In this version, she is Dutch, and she wants to find the fairy prince to warn him about her meadow being flooded by the rain.