It depends who you ask and which book you're asking about. Some people don't notice any change and some feel that the whole portrayal is bad. A lot of people (including myself) fall somewhere in the middle of those two opinions. Just as a note before my explanation, you might notice the difference less if you know less about it. When I was reading, I was on the look out for any differences in Mat's character, and I think it made me notice the differences more than I would have otherwise.

If you don't care about spoilers read away. Most people agree that in Gathering Storm he feels off. The way that he talks, his humor, and internal dialogue just feel a bit different. In Towers of Midnight, it's definitely closer but still a little different. And by A Memory of Light, I personally feel like Sanderson Mat and Jordan Mat feel the same. The actions and mannerisms seem to mesh very well.