score:2
In the ideal case, your itinerary through the airport will be:
This requires you to have a valid boarding pass in hand (either via mobile check-in, earlier online check-in, or maybe at a check-in desk in Ankara) and not be required to go to the check-in desk in Istanbul. I know some airlines require passengers to go check-in desks for Covid documentation verification and have disabled online/mobile check-in for a least some destinations, I have no idea if Pegasus currently do that.
There are however both a transit desk and a "Pegasus Information" desk next to the international transit passenger security which should be able to provide the same services as check-in desks. The only issue is if they apply the regular check-in deadline in that case (see below).
This should take a lot less than 1h40, so if there are no delays you should be perfectly fine. The only complication in this case would be if you arrive at and/or depart from a remote gate (taking a bus), which always adds quite a bit of time.
However, if there is any delay, then you don't have much margin. Depending on the terms of your ticket, you may be considered a no-show, your ticket cancelled, and have to book and pay for a new ticket on the spot (at last minute prices, and provided there's availability). If the next available flight is the next day, any hotels, meals, transportation, etc. would be on you.
If for some reason you need to go through the check-in desk, then the path is slightly more complex:
The path is only slightly longer, but the important point is that you have to be at the check-in desk before the check-in deadline, which is 1 hour before departure. That only gives you 40 minutes to get there from your plane, which starts to become quite tight IMHO.
The usual rule of thumb is to have about 4 hours between two self-connecting flights. More if the second flight is expensive and/or infrequent (e.g. daily or less) or it's the last flight of the day, but it's up to you to evaluate whether you want to take the risk or not.