It's especially hard to express "happiness" across languages. It's a word that is hard to define and generally has no perfect synonyms between languages. It ranges broadly from "present feeling of contentment" to "ultimate feeling of fulfillment in life," and it seems like the survey is aiming for the latter aspect. Therefore the ladder analogy is a decent way to communicate that.

An obvious issue with the metaphor that comes to mind is that if you consider yourself to have a pretty good life, to be overall happy and satisfied, but you think it's possible to have an objectively much better life, then you'd rank yourself relatively low. And vice versa, if you think your life sucks but it could be much worse you'd rank yourself relatively high.