The first of the two main ingredients of a drink that affect the severity of a hangover is obvious. The higher the alcohol content, and the faster you drink it, the worse the hangover. Mixing drinks needn't necessarily increase the overall amount of alcohol consumed, but it may do with cocktails.
No, diluting the volume of the chosen alcohol with lemonade (or coke or water), does not reduce the effects of that volume - it dilutes it - perhaps making it easier to consume but youget the same effect - plus a fuller bladder, sooner.