If you have an idealogical reason not to (like majority of the users in privacy based communities) then you already have your answer and you can read them in the comments.

I'm going to be different here and answer from a practical perspective and say that sigh yes it's probably fine to use WhatsApp if you already use Instagram.

By this point Meta already knows more about you then you'd probably feel comfortable with. They're very good at micro analysing every little signal you generate when you use their platform. I think it's somewhere in the realm of $30 per user per year they make off the average Instagram user. That's the value of how much data they have on you.

WhatsApp users are not monetized so there's less incentive to track you to that extent. Also, WhatsApp messages are mostly end-to-end encrypted. You are giving up your phone number but they probably already have it.

If it were the other way around and you already used WhatsApp but not Instagram I would say NO. But as it is, WhatsApp does not represent a meaningfully significant decrease in your privacy if you already habitually use Instagram.