Looking on the internet deeply has found these results:

She is good in dancing is the most popular phrase on the web.

She is good at dancing 1370,000 results on the web	She is good in dancing 1502,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Andy sings beautifully, and he's really good at dancing. My brother's good at dancing, whereas you're good at other things. Suddenly you got really good at dancing. Suddenly you got really good at dancing. I thought the main risk was it would just make me too good at dancing. I'm not good at dancing, and I'm not as pretty as Louisa. Honey, I'm no good at dancing. I've just always been really good at dancing.	I was able to afford an expensive singing teacher for Veda and a good dancing school for Kay. You're going to make a very good dancing teacher. Suddenly you got really good at dancing. She's beautiful, good in singing and dancing and could be considered the ideal partner. I may not be good in dancing but I'm good in making a sad girl happy. I spent a lot of good years dancing, too. Have a good time dancing, guys. Honey, I'm no good at dancing.

She is good at dancing

1370,000 results on the web

She is good in dancing

1502,000 results on the web

More popular!

I was able to afford an expensive singing teacher for Veda and a good dancing school for Kay.
You're going to make a very good dancing teacher.
Suddenly you got really good at dancing.
She's beautiful, good in singing and dancing and could be considered the ideal partner.
I may not be good in dancing but I'm good in making a sad girl happy.
I spent a lot of good years dancing, too.
Have a good time dancing, guys.
Honey, I'm no good at dancing.