You can, but it may not be the ideal approach. If you are shopping for a replacement hot tub cover, chances are good one of the reasons you need a new cover is because your old one got waterlogged, torn, or keeps blowing off your tub.
The reality is that a thicker, heavier cover is often ideal, so long as it is sturdy enough to hold its shape. Sufficient foam density can help ensure this. There are some products out there that strike an ideal balance between lightweight materials and sturdy, stable design–it is within that range that you should aim to shop.
If you find your new cover too heavy to comfortably lift on your own, you are not alone–this is a common complaint among hot tub owners. A simple solution is to get a cover lift or roller system to go with your deluxe cover. It can make it fast and easy to lift or lower your cover, even if it is made of heavy-duty materials.