Starfleet isn't human dominated, we've just seen a handful of crewmembers from a handful of ships. With 150+ aliens in the Federation, humans make up only a small percentage of Starfleet personnel.
Wrong...
Starfleet =/= The Federation.
The Federation has 150+ alien members. But each member has their own ships/fleets.
Starfleet is Earths fleet, the organization allows aliens, but it is primarily a human organization made up of more humans than any other single alien race. The percentage of humans on that ship may be lower than the percentage of aliens, but when you seperate that into per-species...humans are clearly the most dominant. And you have absolutely NOTHING to base your statement on otherwise.