Headers are essentially metadata and you need some way to indicate that they are separate from your content, but related to it.
I would recommend against lowercase and normal weight text as it will make that row visually similar to the rest of the content.
Title case is the most common, though you might be able to use small caps or uppercase headers. However, these are often less aesthetically pleasing.
If you want something other than bold headers you can use any number of variations including color, alignment, images, borders, etc. as long as the header row is distinct from the content in some way.
The common usage of title case and bold text is a way to make the header row visually distinct with the least amount of visual clutter or interference.
There are many examples of ways to improve tables, but if you look at the ones on pages like http://uxmovement.com/content/9-design-techniques-for-user-friendly-tables/ they all have very obvious headers that are not easily confused for the content.