Last year the bell peppers I grew tasted very bitter but grew to a normal size and the plant itself appeared healthy. I used Miracle Grow every 3-4 weeks.
Bell Pepper Taste Bitter
Certified GKH Gardening Expert
There are actually a few things that can cause bitterness in peppers. One is the soil. I would have it tested to check its pH level as well as any deficiencies. If the soil is too acidic, it may lead to a more bitter tasting pepper. If the soil is low on potassium, this can cause bitterness as well, especially if there has been lots of rain lately or if watering too much--this can leach potassium from the soil. In addition, picking them too early can lead to bitter peppers. Allow them to remain on the plant a little longer for sweeter tasting peppers. Cooler than average temps can be a factor as well.