No, it's not weird to say to a casual friend, "I was just checking up on you." In fact, it can be a thoughtful and caring gesture to show that you care about their well-being and are thinking of them.
Checking up on someone, even in a casual friendship, can help strengthen the bond between you and show that you value the friendship beyond just casual interactions. It also opens the door for them to share if they're going through anything significant or need someone to talk to.
Communication and showing concern for others are essential aspects of building meaningful relationships, and even in casual friendships, expressing care and interest in their well-being can make a positive impact. So, don't hesitate to reach out and check on your friend. They will likely appreciate your gesture.