Most people who ride in hot air balloons stand up during the hot air balloon ride so they can experience the sights much easier than they could when sitting down.
But if you're able to bring a seat with you in the hot air balloon then yes you could sit down in the hot air balloon.
You can also just sit down on the hot air balloon basket floor if you want too but if you did then you wouldn't be able to see the sights.
However if you just want the ride in the hot air balloon then sitting down in the hot air balloon basket is fine but if you want to see the sights from the hot air balloon ride then you'd need to stand up.