No, I'd say that's normal for those heaters,
I've had them in 5 gallons all the way down to 1/2 gallon sizes,
And as I said it will reflect the temp of the room they are kept into.
Ideally it would reach 78,
74 is far better then 64 and better then having a heater that may cook him...
My kids had several in their bedrooms that only reached that temp and the betta did just fine,