Yes. The GNU General Public License explicitly allows for charging for redistribution (which you’re not actually doing in this case):
You may charge a fee for the physical act of transferring a copy…
…and explicitly allows for unlimited usage rights:
Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted…
Note that there are already several commercial services based on providing WordPress as a service, including Automattic (wordpress.com), WPEngine, and others.