Is Mojito slang for 'little Mojo'?

Umm, yes.

'It is rumored that the origin of the word "mojito" is derived from the diminutive of the word mojo.'

'Although mojito has no true translation, supposedly it originates from the root mojo. The suffix -ito in the spanish language means "little". So possibly, the word means "little mojo" or something to that matter. Many summer cocktails contain sexual references.'

https://en.wikipedia.org/wiki/Mojito#History

From Merriam-Webster dictionary:

Origin of MOJITO
American Spanish, diminutive of moje, mojo citrus marinade, from Spanish mojar to moisten

http://www.merriam-webster.com/dictionary/mojito