Yeah, the cheap ones are basically Lilos so you need something that stops air flow, i.e. insulation. Unlike sleeping bags which have an upper comfort limit, mats can be warmer than you might need so get the best insulating one you can afford then if things get a little nippy at night you’ll be fine.
If you are car camping then weight and bulk aren’t that much of a concern. Self inflating mats tend to be a bit heavier than the ones you inflate because they are full of foam that’s a bit like a sponge. Compare the Double Dozer at 5Kg with this Exped at 1Kg https://www.ultralightoutdoorgear.co.uk/equipment-c3/sleeping-mats-c58/airmat-hl-duo-m-regular-double-sleeping-mat-p9405, twice the price though.
There was a thread on here where people were extolling the virtues of the Exped Megamat but it’s not one I’ve used.