If it is light and still has the RR configuration it would be a pain in the butt to control if you drive in a spirited manner, since RR cars are tail happy.
That's over-simplifying things a little.
The sort of RR cars you're describing typically have the engine slung out over the rear axle by a healthy margin - older 911s, Skoda Estelles, old Beetles (and other aircooled VWs) etc.
In Smarts, the engine sits on, or even slightly ahead of the rear axle (this one is the older model, but indicative of the layout):
In other words, it doesn't have a big pendulous weight following it around the entire time. Its centre of mass is between the wheels somewhere.
In previous Smarts, there have also been other, much more significant barriers to being able to go quickly - a narrow track, short wheelbase, skinny front tyres to induce understeer rather than destabilising oversteer. If you drive a current Fortwo Brabus quickly and aggressively you also get an over-active stability control kicking in all the time to ensure you don't overdo it. (It's worth noting that if you deliberately drive smoothly it's possible to go quite quickly in the old Fortwo - it just needs technique).
The latest Fortwo has a longer wheelbase and a wider track. Having sat in a largely similar (if longer) new Renault Twingo, it also has a lower seating position, which I assume is indicative of a lower centre of gravity. All those should make it quite significantly better to drive than the current one. Throw in the inevitable stability control and oversized rear tyres and it's hardly going to pirouette around like an old swing-axle Beetle whenever you start driving quickly.
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