, diarrhea is not a super-common side effect of prednisolone, but it IS a possible one.
Prednisolone is a steroid medication (basically, prednisolone is absorbed by the body with more ease than its precursor steroid form, prednisone) and steroid medications can have many side effects.
My 17-year-old kitty has been on prednisolone for a number of years for her irritable bowel syndrome; her dose is 2.5 mg every other day. She has never had diarrhea as a side effect. However, her dose isn't as high as Princess'. Is Princess supposed to be on the prednisolone only for a short time, or is it a permanent medication?
Also - I am a bit surprised a vet would prescribe a steroid for an ear infection and not antibiotics, but I'm not a vet so maybe there is a reason for the prednisolone prescription. Prednisolone, as a steroid, is anti-inflammatory and can also help allergic reactions, so perhaps the vet thinks there is an allergic component to the ear condition, or perhaps he only prescribed it as an anti-inflammatory.
I wouldn't change Princess' diet over this, as abrupt diet changes can cause even more stomach upset. I'd especially leave her on a wet food diet, but that's more my personal opinion (I hate kibble for the health problems it causes).
As an aside, prednisolone, like most steroid medications, suppresses the immune system, so keep Princess away from possible sources of infection (other cats, the outside, other animals, etc.) until she's off the prednisolone.
Also - my kitty has been on Revolution for years and has never had a side effect from it. She also is quite old (17!) and has many medical conditions, including kidney failure, IBS, hyperthyroidism, and high blood pressure, so I figure she'd be a prime candidate for side effects if any kitty was going to have any ;} She has been just fine on a once-monthly dose of Revo for years.
But all this aside - give her vet a call and tell them about the diarrhea. Be sure you make it clear that you are concerned and this isn't a case of "oh she just had one runny poop". Make them check Princess' records to remind them of all the meds/treatments/vaccinations she had, so they can accurately decide whether the diarrhea is a concern, or if it will pass in a few days.