While both are polyethylene foam, mincell is "microcellular" and the camping pad material (Ethafoam) is "macrocellular". The larger cells trap water more readily - especially when the outer skin is removed - and reduce the surface area for gluing. These foams don't glue as well as minicell (commercial Ethafoam laminates are heat fused) and as Jay indicated, they don't carve well at all. They are also much less resilient. The lighter densities of Ethafoam (1.2# & 2#) are not durable and the heavier densities (4# and up) are too hard for kayak seats. Minicell is really the way to go.

If you're trying to save money, there's a simple way to do it. Instead of buying expensive 3" foam, carving it to shape and throwing much of it away, build up a seat with pieces of 1" foam placed only where you need it. The end result is the same, but the cost is lower, as is the amount of waste. Contact cement works great for laminating minicell, my favorite being Weldwood Gel.