It is the thermostat opening, so you are correct.
Really depends on driving conditions to tell if it is normal.
When I leave my house I have to go up a longer grade on the highway after about 4 or 5 minutes of driving.
If there is traffic, so it takes longer to get on the highway, then thermostat is just opening at the top of the grade so temp will go up to about 3/4 on the gauge and then drop back down.
If there is no traffic and I get over the grade faster then temp will go up just over 1/2 then drop when thermostat opens.
After thermostat is open for awhile temp gauge runs just under 1/2, but will climb to 1/2 on longer uphill grades
Been that way for years.
If it was doing it on flat roads then I might change thermostat, really up to you
Good read here on changing thermostat on the 4.0l SOHC: How To Replace the Upper and Lower Thermostat Housing on a Ford 4.0L V6 SOHC Engine