Is a bootstrap capacitors always required for mosfet drivers?
The datasheet shows this for a reason.  If you are not completely sure what it's purpose is and know that you don't need it, then you have to include it.

In this case, the FET driver is intended for N channel FETs both top and bottom.  When the top FET turns on, it's drain and source voltage will be close.  After all the purpose is for the FET to act like a switch.  N channel FETs require a higher gate voltage than the source to turn on.  This means a higher voltage than the top rail.  The diode along with the cap between Vb and Vs are part of a charge pump to produce this higher voltage.

If you happen to have the higher voltage already available, then you might be able to use it and not include the charge pump.  To do so, you need to read the datasheet very carefully and see if this is possible with that device, and if so, exactly what requirements and restrictions there might be.

One drawback of the charge pump method is that the FET needs to be regularly switched for the charge pump to maintain a voltage ready to switch the FET.  If you will have long periods at 100% PWM, for example, you may have to consider other methods.  Again, the datasheet for the FET driver should go into detail on all this.