As other answers indicate, it's quite straightforward that in these specific cases, the only acceptable "question tag" is isn't it? It would be much more incisive (and more awkward) if OP's example had been:
It's important that we should eat healthy food, ...........?
That still wouldn't change the fact that technically speaking, isn't it is still the only "correct" tag, but in practice many/most native speakers wouldn't see anything odd about using shouldn't we there.
As an example of how ridiculous things can become when people try to force "strictly logical" constraints on the "subject" of a question tag, consider this sentence:
All I meant was that he should have seen it, shouldn't he?”
Those very same "strict rules" that require isn't it for OP's example would force us to accept that the only valid tag for my example must be didn't I. It probably wouldn't raise an eyebrow among native speakers if you did use the "valid" form, but unquestionably the version as written would be far more common.
In short, there is a vague area hereabouts, since meaning affects question tag format as much as, if not more than "grammatical rules". In most cases, the question tag agrees with the initial subject/verb. But if there's a more semantically significant "secondary" possibility, it's often acceptable to target that instead.