It isn't really necessary on the smart, IMHO. The plastic roof and plastic body panels are pretty much immune to hail. Even the metal tridion would be at such an angle to falling hail that it would be tough to damage it. If the hail is big enough, it will take out the windshield regardless.
Reminds me of an exciting happening some years ago.
Ann and I were attending the annual state car club weekend meet, show-and-tell and judging at a very nice and near-rural motel near Austin, Texas. Come Saturday afternoon we were sitting on a terrace enjoying our first cocktail of the evening, when the sky turned black and rumbling, with lightning flashing here and there (mostly here), and a fierce Norther blew in.
To our amazement we watched the other attendees toss their bedding off their beds and haul their mattresses off to put them on the top of their cars, having experienced before the devastating hail often seen in that part of the world. So Ann and I followed the advice of the old-timers and did the same, just moments before we were deluged with hail ranging from the size of golf balls to that of baseballs.
These cars were several millions of dollars worth of Rolls-Royces and Bentleys -- vintage, historic, modern -- all out in the open. No complaints from the motel hosts, as this annual event had seen this happen before.
When living in Colorado we often saw hail ping-pong ball to baseball size. A work buddy had an aluminum pickup topper punched through in several places from baseball size. Sometimes cattle would be bruised and bloodied from the awesome hail there... and they couldn't get away from it...
With the smaller stuff, it would clog up the storm drains and snow plows would have to come out to clear the streets!
I would think any sort of cover to lessen the impact should help in some way...