Monkeypod is definitely walkable - it's maybe a 15 min walk up a hill (that has a walking path and is not particularly steep) and well worth the walk / incline for the amount of mai tais and beers you may consume. I couldn't imagine driving there from any resort in Wailea, except perhaps the Fairmont or Hotel Wailea.
And, to your question above, you need a car on Maui. I couldn't imagine not having one. Some days we never touch it in valet and just hang at the resort, but even factoring in the cost of that, the convenience far outweighs that benefit. Further, as it seems like you are quite cost conscious, a car allows you to easily stop at Walmart/Target/Costco and stock up on snacks and drinks before arriving to the hotel. This saves us a lot of money, particularly on the booze front.
Regardless, and I know I'm speaking for myself personally here, but trying to pinch pennies while in Hawaii will ultimately make your trip more stressful. Hawaii, albeit absolutely beautiful, isn't that relaxing (at least not like it used to be), so adding stress on to your vacation worrying about the self-parking cost is honestly not worth it, IMO.
Also, when we were there in Dec 2020, we had to pay for valet, but only self-parking was allowed - is that not the case anymore?