Short answer- no.

Longer answer: no, because hippos are aggressive and not really suitable for the purpose of riding.

Hippos are not domesticated animals and are not nearly as trainable as camels, elephants, and horses. I see questions like this on this sub a lot- people want to know if X culture ever rode Y animal. The answer is almost always "no."

The only critters that are really practical to ride are the camel, the donkey, the horse, the mule, and the elephant. Even then, donkeys can be troublesome.

There is a reason for this: these animals are trainable. You can teach the animal to reliably respond to cues. Except for the elephant, these animals also have a mouth that allows for the use of a bit, which is integral to controlling and steering the animal.