I know there are certain limitation on how many private investors a C-corp can have. Would these people putting in money via kickstarter be called as investors?
Kickstarter cuts you a check via Amazon. You don't have any contact with the donators except as mediated via Kickstarter. There's not even a requirement that you fulfill your promised obligations to your backers (although they'd probably never let you use them again).
People are literally giving you money out of goodwill.
That said, Kickstarter is a terrible platform if you don't already have a social network into which you can cast a wide enough net. It works best as the way you enable all the friends you already have and who are already willing to give you money, to do so.
It doesn't have a huge pile of people who are just looking to spend money on hairbrained startups with no return. That active curation, cutting out projects like that, is why it's succeeding.
I however disagree that it isn't worth doing if you aren't already popular. You can use it as a place to become more popular than you already are, or use it as a means to show some traction even if you only raise $10,000.