If you think someone can get better at something they’re doing, definitely tell them. If they have the right attitude, they’ll appreciate the effort, but be careful that you don’t embarrass them in front of their peers while doing it.
It is especially important to give negative feedback in private. It might seem obvious, but I know I’ve been given negative feedback in front of others in the past, and it’s not pleasant.
If you’d like to use this opportunity to learn more about how to give feedback, I did a bit of Googling for the topic, and I was happy with the results for this set of keywords: giving negative feedback to employee . Let me know what you think in a comment!