It is almost perfect weather in March particularly the first half but for this year the second half seems likely to be as good. The Mughal gardens is a year which you can miss dafter 15 March. personally, however, the sky shades and weather wise it is the months of September and October that I prefer, just before it turns into the smog capital of the world.
It is a good time to visit Delhi. It is a pleasant weather as of now and is getting slightly hotter day by day but till the month end it should be fine.
The whether is not so cool and not so hot. You can travel in a comfortable light jacket. Tourist attractions are always crowded but it depends on the time of the day which you will visit places.
It will be pleasant during march end a pretty good time to roam around all the historical places and checking out the by-lanes of delhi.... but delhi being delhi will be crowded as usual....