Or some sort of shirt in general. I could see a nice button down being used to add some nice texture and pattern. Either way, there should at least be a shirt. It's truer to his character's story.
For justification, The Lizard isn't just Killer Croc in a lab coat and pants. He is a man (Doc Connors) who morphs into The Lizard when he loses his cool thanks to self-testing of a serum. So, I don't think you'd ever see him rocking a lab coat and pants without a shirt on underneath because he's usually in human form.
Unless he is "playing doctor", then I'd be shocked if he was wearing pants