I get what you mean - the seat is quite slanted on the Joie Pact line (and, unfortunately, on most ultra-compact strollers. It is given by the fact that the pushchair is made with one goal in mind - to be the smallest possible, meaning compromises in most other areas since the stroller's priority was to be the smallest possible.
The bad news is, you won't help yourself much. There is this thing you can try - and that is, to put a belt or any kind of strap behind the backrest and secure it around the bars under it to hold it more upright. I am not sure though that the small frame of such a compact stroller will allow it. Some buggies allow to rip the strap belts behind a bit to make it possible to pull it more (but I wouldn't risk it, it is possible mostly with prams like Britax B-Agile...
Other than that, there's not much else to do - except for changing the stroller. I mean, if your daughter hates the position she's in (I mean, when too slanted, the child does "hang" on the harness and that is not comfortable, you wouldn't like it either). It may change, but it probably won't... so the only answer is to change the stroller. You will see how she reacts differently. There are not many upright ultra-compacts, I can only think of a Chicco Goody... I would also consider getting a lightweight mid-size stroller, they're usually much comfier for the kids (think something in the size of a Mutsy Nexo... They don't even need to be completely upright strollers, actually - just the different anatomy of the stroller's seat will make a change, you'll see.