This is is the exact question I want to ask about half of the people commenting on here...
Including myself sometimes lol
Guilty as charged; we all are, really.
个 or 是?
We don't use 是 here because 'important' isn't an actual object/thing - you could say '这个很重要吗？' and that would be correct.
and why not omitting 个？ 这重要吗？would be correct