FWIW, I haven't experienced any notable degradation of the seats in my 08 (heated leather) with about 65k or either of my 09s (one heated leather and one cloth) with roughly 55k each. I'm a big guy, and my cars are split between Florida and it's inherent heat, and Michigan with the brutal cold winters.
My Michigan 09 developed a small indent from my wallet, but that's about it.

 

I'm also "big boned and fluffy" and my 09 with 0ver 100K has NO seat problems.

My smart is one of the most comfortable seats I've had in any vehicle........it was a pleasure on my last long trip of over 4500 miles, heated seats (for my arthritic back) and A/C blasting !!

 

same here. I call it "maximum density" and no problems with my 2009 with 165K, or with my 2008 with 103K on them.

 

I'm really a small boned individual. The smart cars seats are comfortable for short trips around town. Never had a issues of the seat deforming, after 8 years of ownership. I've driven it many long distance drive. 3 hours is about all I can handle in the car. It seat cushion, are flat bottom. No real lumbar support. Or adjustment for it. The smart is a city car.

 

Have you checked to see what a new seat bottom would cost from MB...?
Also check at a Junkyard for a replacement, they charge 1/2 of a new cost for one...