Kindergarten is time for kids to practice their printing. It's important for them to learn to differentiate between capital and lower case letters and to learn to write them. It's very normal for kids to get them confused, but it's something to work on. Children this age commonly make "reversals" when writing, which is like writing a "b" when they meant to write a "d". These mistakes usually correct themselves over time.
There are also special pencil grips to make it easier for struggling kids to attain a correct pencil grip. These are usually plastic or rubber, and slide onto a pencil to make it easier to hold. They’re inexpensive and definitely worth a try. There’s also something called a “heavyweight pencil” that can help kids to focus their attention on letter formation. It’s a mechanical pencil that is actually weighted to help kids learn better writing control.
If all else fails, an evaluation by an Occupational Therapist may be in order. They can check your child’s motor skills and eye-hand coordination much better than you can at home, and often recommend helpful strategies to solve the problem.