Would Nigeria be less corrupt if Jonathan
wasn’t president?

Yes, if the right leader with the better motive towards fighting corruption in the country is being appointed and also if the anti-corruption agencies are being allowed to carry out their duties without interference.
In summary i.e. If the right leader is appointed and the right things are put in place.

Maybe!!!
but more corrupt if Buhari is

the only way u can be sure is if u can see what could have been.
But come to think of it ppl fail to see that corruption is not only abt our leaders. The populace in the own small way are also guilty.
From ur topic there is also a possibility that if another person was president, corruption could have been worse

YES. You need courage to oppose corruption. You also need a lot of sacrifice. No be ordinary eye Mata. Some of your closest friends, people you love and respect would be involved and you need to make the law act without interference.