Probably too much info, but.....
Most On/Off thermostats are solid state (electronic) therefor are "soft start" meaning that they turn on at the precise moment that the sine wave is at it's lowest point.
This stops all "inrush" spikes, The spike that kills light bulbs when you flip the switch at just the right moment. (Ever notice that bulbs always go out just as you turn on the light??)
In other words, the start up cycle is non damaging.
Your heat mat will be fine....