Did she have to go to the park to make the strip read correctly?
Was it Pearls Before Swine? They are always doing crossovers.
No.
The comic strip in question is not given.
Was the comic strip missing something that was present in the park?
reply to Cheesesteak:
Was the comic strip missing something that was present in the park?
No
Did the meta comic strip take place in a park?
Did the meta comic strip mention a park?
reply to Chronos:
Did the meta comic strip take place in a park?
Did the meta comic strip mention a park?
We do not know anything about the topical content of the newspaper comic.
Do we know anything about the graphic content?

The comic strip in question is not given.
What I meant was was Nancy in a Pearl Before Swine comic where this occurred.

Do we know anything about the graphic content?
We know nothing about what the strip portrays.

What I meant was was Nancy in a Pearl Before Swine comic where this occurred.
It was an Ernie Bushmiller regular Nancy strip. The strip in the newspaper is unknown and irrelevant to the gag.
Is the time of day relevant?
Day?
Night?
Sunrise/sunset?
reply to cheesesteak:
Is the time of day relevant?
Day?
Night?
Sunrise/sunset?
Day. Relevant.
Summary:
There is something wrong with the way the strip is printed.
We do not know what the strip portrays.
It is relevant that it occurred during the day.
Was the strip printed on the wrong page? Upside down? Vertically?
Does the strip or gag involve the sun?
I’m flailing around like one of those wacky inflated tube men at a car dealer.
reply to Cheesesteak:
There is something wrong with the way the strip is printed. Yes
We do not know what the strip portrays. Correct.
It is relevant that it occurred during the day. Not the newspaper strip. It is relevant that Nancy went to the park in the daytime.
Was the strip printed on the wrong page? No.Upside down? Yes. Vertically? No
Does the strip or gag involve the sun? The gag. Must be daytime for it to work.
Does the gag involve shadows?
Does Nancy go to the park and hang upside down from a tree or the monkey bars? Not sure what the gag would be, unless it has to do with shadows like @dirtball suggests.
Oh, the gag might be that she hangs upside down from the monkey bars while she holds the newspaper right side up so as to read the comic strip correctly.

Does the gag involve shadows?
No.