It's hard to pinpoint a single 'strongest stand' in Jojo manga. Different stands have unique abilities and advantages. For example, The World has the power to stop time, which gives it a significant edge in many situations.
Unique character design. If the original character (OC) has a distinct personality and backstory that fits well within the Highschool DxD universe, it can make the fanfiction stand out. For example, an OC gamer who has a special power related to gaming that also has an impact on the DxD world in an unexpected way.
Well, first of all, a strong character arc for the MC. If the MC starts from being weak and gradually becomes OP in a believable way, it's really appealing. For example, in some novels, the MC has to endure hardships and learn from his experiences to gain power.
There's no way to really tell if it's common specifically for someone named Jojo. However, in general, reading fanfiction is a popular activity among fans of many different franchises. The Jojo series has a large and dedicated fan base, so it's likely that there are fans named Jojo who partake in reading fanfiction, but we can't say for sure how common it is without more data.
In your 'Seven Deadly Sins OP OC Fanfiction', you can make it stand out by having a distinct writing style. Use vivid descriptions to bring the world of the Seven Deadly Sins to life. Describe the battles, the magic, and the different kingdoms in detail. Also, make your OC's power have a cost. For example, if they have an OP power related to pride, every time they use it, they lose a bit of their sanity or some other negative consequence. This adds depth to the character and makes the story more interesting. Moreover, explore moral and ethical themes through your OC's actions and decisions, which will engage readers on a deeper level.
Perhaps this Jojo is interested in exploring different interpretations and stories within the Jojo universe that fans have come up with. Fanfiction often takes the existing characters, settings, etc. from the original Jojo series and creates new and unique storylines. So, this Jojo might be a big fan of the series and wants to see what other fans have imagined.
It depends. Maybe there are some telltale signs or elements that suggest it's a JoJo reference fanfic, but without specific context, it's hard to say for sure.