If you think about what you want to achieve, and you put time and effort into the end product to make it as good as you possibly can, then you shouldn't feel embarrassed, even if the end product isn't objectively very good, because you tried your best on it.
There is a tendency, however, to have an idea, immediately go off and design some elements of it, put it all together and say "there you go, finished", but no mock is impressive because of how quickly it was put together, it's quite the reverse. Spend time trying out different things. If something doesn't look great, don't think 'ah well, it's done now', think 'I'll have another look at that and try and make it better before I finalise it'.
There are a range of brilliant mocks on here which you can look through for inspiration, or places like Behance to see how real-life designs are brought together, from choosing the typeface, to the colour palette, to the shapes and sizes used. You can take inspiration from all over the place, and seeing a range of other good designs will give you tips on how you can improve, as can, of course, receiving feedback from your peers on places like this.
I am far from an expert myself, but if you want to talk about this in more detail, or you just want to have a chat and throw some ideas around, I'm more than happy for you to send me a private message.
Guess who's big in the back time.