Yes, it is okay for a guinea pig to eat cherries but only in moderation and not too often. As with many different fruits cherries are high in sugar content and too much sugar is bad for their diet and could make them very ill. Therefore only give cherries as a rare treat to your guinea pig and they should be fine.
The positive thing about cherries is that they are a good source of Vitamin C but on the downside they are also full of sugar.
Therefore as mentioned above guinea pigs can eat cherries but they need to eat them in moderation otherwise it could cause digestive issues, weight gain and possibly other more serious health problems in the future.
One other thing to consider is that you will need to remove the stem and seed of the cherry before giving to your guinea pig. As well it may be a good idea to chop them up into pieces to avoid choking hazards.
As well you should thoroughly wash the cherries to make sure there is no pesticides or any other chemicals on the outside of the cherries.
A small amount of this fruit once or twice a week at most is a good portion size. Anything more is not necessary for their diet.
As always the most important thing to feed to a guinea pig is fresh hay and water. As well fortified pellets with Vitamin C in them every day is a good way to keep your piggy healthy.