Is there any sort of mini heater that can be powered from a powerbank to be used in a tent (or anything similar)?
I did not realize a powerbank was a phone charger.
Even a car battery at 6 amps (that would last like 8 hours) is about 70 watts. A person puts off about 100 watts.
Even a car battery would not be much of a heater. Heat is an intense form of energy.
I think you would get about 5 watts from a powerbank and run 8 hours. Even high capacity is 10 AmpH and the voltage is 3.7.
They have battery powered electric blankets. That is probably a better option as a tent is not not a good insulator.
If you mean by powerbank something that weighs 200g and has a capacity of 20000mAh and by heat more than a candle worth of warmth, then the answer is no. A 20000mAh powerbank, which is a pretty big phone charger, can source 100 Watts for an hour (ignore for a second that it cannot safely discharge at that rate). This answer tells us a candle produces 77 Watts, so a powerbank is like burning a candle for 75 minutes. Space heaters are around 1000W, so you could run a space heater for 6 minutes.
Even if 100Wh was enough for your heating needs, powerbanks are designed to discharge at around 4A (20W), so you would only be able to produce a little heat for a longer time.