No so long ago I asked for the performance of C172 in this post:
2020 172 or 2024 172? - User Support Hub / Aircraft & Systems - Microsoft Flight Simulator Forums
There you can see a link to a document with the real performance figures of the C172. According to those figures, it’s very difficult to climb to 12,000 ft with this airplane.
In the sim I have also experienced what you say. In career missions I’m selecting no more than 9,000 of cruising altitude, and still it’s difficult to get up there.