Different people use different terms for the two concepts I am about to outline ... and I thought I should post up a page so readers can keep straight which I am talking about when I use the words "mockup" versus a fitting muslin.
I use the word mockup to describe something I hope to end up wearing. This is sometimes called a test garment or a wearable muslin (although I will never use this last phrase) and I will often us cheaper fabric bought before I knew what good quality fabric was - and what I preferred. My purpose of making a mockup is to test a pattern, its instructions, new techniques, and ultimately how the style actually looks on me since I do not have the same body shape as the artistic sketches or even the models on the pattern envelopes.
If a mockup works out, then I have a new garment to wear ... if it doesn't work on me due to style, I will be donating it or giving it away. If I hose up a new technique beyond fixing ... I won't be too upset about the fabric loss, although as usual my pride will be bruised.
This is never intended to be a finished garment. I will often recycle a wadder or use scraps when possible, or make it out of cheap cotton muslin or canvas or an ugly $1/yard cotton as long as it will show Sharpie marks. I usually sew it in contrasting thread, and will often use a long stitch length so I can rip it apart and make tweaks. In my little sewing world, a muslin is never intended to be wearable ... it is to nail down fit issues.
Once I get fit issues nailed down, then I usually pull the fit muslin apart, starch the everlovin' **** out of the pieces and use those as my pattern pieces, if they haven't frayed too badly. If they have frayed beyond use, I use them as templates to draw new pattern pieces onto extra tissue paper or my red dot stuff.
Quick summary: muslins are for marking up and taking apart. Mockups get made start to finish, and in hopes of a wearable garment as the result (for someone).