There is a short summary for MDX , that might be useful to you regarding to the question : mdx cheat sheet
And for dax it is DAX Cheat Sheet
Is this any interview related question ? We could not say any queries are more important than other , just like you would not say there is some important SQL queries or C# codes. If you would have basic knowledge of MDX and DAX, Joyzhao's post have give good direction.
Another thing is that, MDX and DAX is not mandatory in SSRS. MDX is also very less used in Power BI.