I normally see some crypto projects which promise Investors certain amount of dividend depending on how much coin/token they own.
The word "Investors" is normally used for people who own shares of a particular project/company, but in cryptocurrency this "shares" normally refers to coins/tokens, which makes me wonder if bounty/airdrop hunters are also considered "Investors" since they hold certain quantity of these coins/token
Technically, yes. But i would say, it depends.
If the bounty hunter holds the rewarded coins then i would say yes. He invested his resources (albeit, time) into the project and will get a reward for it. But then again, it depends on the bounty. Does the bounty pain in tokens or in some other fungible reward? What the reward is and what happens to it would give you a better idea.