Is "On XXX of YYY" a good thesis title?
The title of a work is the first thing that people will use to decide whether to read it or not.  You want it to say as clearly and lucidly as possible what it is that you have contributed.  If you title something "On XXX of YYY", then that says you've done a loosely coupled collection of investigations regarding the relationship of XXX and YYY.  If, however, the important conclusion of your thesis is more specific, it's good to say that in your title.

To make up an example, compare:


"On the diet of Antarctic seals": this document studied diet exhaustively, and came up with very thorough data but no particularly big surprises.
"Antarctic seal diet dependent on frozen pizza": this document studied diet, and found something rather bizarre that clearly needs more study.


In sum: short is good, but unnecessarily vague is bad.