Bounties require you to do work. In this regard, they are not a source of passive income (by definition passive income is what you receive for an investment or an asset, without having to work for that money).

If you are not selective and you do participate in any bounty, then yes, waste of time.

If you do bounties for good projects that you are investing yourself, which is what I do, it is a great way to get additional tokens for free and lower your investment costs.

But better projects out there are not doing bounties anymore, they do telegram airdrops or other kind of campaigns. Those are worth cents, so for me a waste of time. You need to select carefully.