What should be the design or shape of a rocket going to mars?
I think you already know the answer to this, as taking a look at any rocket planned to go to Mars shows the same basic architecture.
Especially for Space-X's technique of landing atop rocket thrust, it doesn't matter so much what the aerodynamics are for Mars, so the shape is driven in the main by launch requirements from Earth:
Lots of fuel, and discardable stages lead to the classic "rocket" shape with the important cargo (humans and associated life support etc) at the top away from the hot end, and any stages which need to be dropped.
As you rightly pointed out, Mars' atmosphere is very thin, but aerodynamic surfaces will still have an effect, which is one of the reasons they are present on the Starship design.
No major changes in Rockets which is planned for MARS mission. You can see in STARSHIP. It still looks like Rocket but yaa MARS Atmosphere very thing as compare to Earth so Yaa you can add more fins same as STARSHIP. Because in Cruising flight if more surface area is presant in Rocket then it will be more Reliable for decreses the Decent speed.