There is no way to simply exclude source controlled DLLS from Code Review.
Daniel is right, for this kind of external dlls, the best and clean way is via the package management solution such as NuGet. There is an excellent extension called-- Package Management in TFS. It's not hard and will not take too much time to do this.
Package Management is an extension that makes it easy to discover, install, and publish packages. It's deeply integrated with other Team
Services and TFS hubs like Build so that package management can become
a seamless part of your existing workflows.
For more details, you could have a look at this tutorial-- Package Management in Team Services and TFS
Besides, there is also not any built-in advanced filter (e.g. exclude comments, white spaces, new lines). The official tutorial for your reference: Get your code reviewed with Visual Studio
You could also try to use some 3-rd party extension in VS such as this one: Review Assistant - Code Review Tool which also support TFS integraton.