In order to provide longitudinal stability the tailplane must experience a change in angle of attack when the aircraft pitches nose up or nose down.
The greater the change in angle of attack, the greater will be the stabilising moments generated (provided of course that the tailpalne does not stall).
A low tailplane tends to be enveloped in the propwash. As the aircraft pitches nose up and nose down, the propwash direction pitches with it. In a nose up pitch for example, the tailplane and the propwash are rotating down at about the same rate. This tends to reduce the amount by which the angle of attack of the tailplane changes. This in turn reduces the stabilising effect of the tailplane.
A T-Tail tends to locate the tailpane above the propwash. So as the aircraft pitches up and down, the tailpane experiences a greater change in angle of attack. So it produces a greater stabilising effect.
So T-Tails tend to increase longitudinal stability.