Mental model and mindset are synonyms but mindset can have a global flavor
The short answer is that a mindset is a mental model. One way to think of this is that the term mental model is like the term ice cream. Each of these synonymous terms are flavors of ice cream: beliefs, concepts, ideas, schema, knowledge, mindsets. The Dictionary defines mindset as follows:
the established set of attitudes held by someone.
This definition provides a clue as to why mindset "feels" a little different than mental model. The clue is in the word "set [of attitudes]" which means a group or a multitude of attitudes. Attitudes—like beliefs, concepts, ideas, schema, knowledge, and yes, mindsets—are all mental models. But when we think of a mindset, we get the sense that there is something "big" or "larger than" a mental model. This is because of the set-like quality of the term mindset. It conjures the idea of many mental models coming together to form some overarching super-mental model. And, this is true of mindsets. They are often generally pervasive in our paradigm or approach to things. Carol Dweck's work on Mindsets has renewed our interest to the term by distinguishing between a Growth vs. A Fixed Mindset and then breaking these mindsets down into (part-whole) Systems and showing the remarkable effect they can have on a person's outlook when these Mindsets are used as a Perspective. In this sense, the effect is global in that the mindset's effects apply to almost everything in one's life. But, it should also be noted that you can have a "mindset" that isn't globally applicable but that is applied to a specific domain. For example:
my mindset toward peaches is that they are too fuzzy and make my mouth feel weird.
A set of mental models is still a mental model. Using our ice cream analogy, if I were to make a pile of different flavored scoops, its still a pile of ice cream. Net-net, mindset is just another word (or flavor) for mental model.