SSTO would probably more relevant to return from Mars rather than to go there in the first place - building a solid booster or building a mars recoverable one might be crazy.
However, the atmospheric pressure in Mars is a lot lower than on earth, which means the same diameter scale issues don't apply.
And the orbital velocity is also much lower, which makes talking about Mars SSTOs kind of weird in the same sense that talking about Lunar SSTOs is rather weird. (If you need a multi-stage rocket to get off of the Moon, I really want to see your mission architecture)
I think Starship is intended to go SSTE (Single Stage to Earth) from Mars, for instance.
However, the atmospheric pressure in Mars is a lot lower than on earth, which means the same diameter scale issues don't apply.