However, most software engineers commonly use the framework that supports the requirements and contract to create a UI. Just like using realistic textual content in your mock up, using the final products toolkit impacts your project in two negative ways that I can think of.
The second problem with using your final product's toolkit for a mock up is one that I have not shared with anyone prior to this post. Consider the emotional feeling when you open some gadget or desired item you ordered online that just arrived at your house after waiting "forever". That new plastic smell. The rich visuals your eyes can dart all over. The inquisitive way your mind bends around everything that is happening. This is what you want your customer to experience. A brand new and fresh product. Not some half done knockoff. Once the customer has seen the 'rough draft' UI for the final product, the cat's out of the bag. You and your team worked hard on this product, don't sell it halfway through. Both the team and customer will quickly tire of the subsequent demos and your product will be old news before it ever hits production.
I understand that every project has unique customer's constraints and requirements. This is by no means an absolute technique for demos and mock ups. But something I think one should consider.