score:7
No, you shouldn't have any trouble.
As you said, the situation in Germany is relatively normal. There are still virus-related restrictions, but there are plenty of flights operating and plenty of taxi drivers who still need to eat and pay rent.
At Tegel, there are taxi stands at Terminals A, C and E, so no matter which terminal you arrive you shouldn't have to walk too far to reach a stand. There is no mention of any coronavirus-related limitations affecting taxi availability on that page or on the airport's general coronavirus information page. When you reach a stand, it's likely there will either be taxis waiting already, or (at particularly busy times, such as when a few large planes have recently arrived) a short queue of people waiting and a relatively steady flow of taxis coming to take them. In the unlikely event that there are no taxis waiting and no sign of any arriving, the taxi page from the airport's website (linked above) lists several local taxi companies you can call.
The fact you're arriving from outside the EU should be entirely irrelevant. You said in the comments you were concerned that drivers might be apprehensive about taking you. The law is on your side, taxi drivers at the airport are required to take you to any destination in Berlin:
Are taxi drivers allowed to refuse to take me to my chosen destination?
Taxi drivers departing from Tegel Airport are obliged by law to take any passenger to their chosen destination within the state of Berlin and to Berlin Schönefeld Airport on request.
They'd need a very good reason to be allowed to refuse you (for example, if you were being violent or abusive towards them), some vague fear that you might be more likely to have the virus because you came from outside the EU is not that.
I suppose it's theoretically possible that a driver might refuse to carry you even though they're not allowed to do so, but I'd consider it extremely unlikely - and even if it did happen, the solution would simply be for you to get in the taxi behind it.