That's a tricky question to answer. In the beginning, you need a decent mathematical aptitude in order to get through the degree. Mechanical in particular, given all the engineering analysis you will be doing. Note that this varies between unis and some people will get on here and say it's easy and you do hardly any. I did a heap of this due to the Aerospace part and it was damm tough and I usuually I find maths very easy. Some unis offer a lot of assisstance, but the one I went to requires you to mainly learn it by yourself. It's a top uni for the profession for a reason. This shocks a lot of people and causes a lot to leave as a result. Probably coming from private schools who hold your hand the whole way through...
The main thing is your marks to help you actually get a job. Unless you have a relative that is an engineer and can get you a position, you're going to be competing with a lot of other people. Most companies use your grades to initially thin applicaitons for interviews. This is when your personal attributes come into play and where you can beat people with higher marks based on this.
In other words, you can get by without being the smarest person in the world, but you need to be competitive in the beginning to get a foot in the door. Grades are even more important now with the small amount of graduate positions being offered.
It also depends on what type of job that you end up doing. Not all enigneering jobs require actually doing any engineering analysis. You might be a project manager or technician supervisor. I'm persoanlly a design engineer and do maths all day. When it comes down to it, you can be successful in any role, it just comes down to finding the right position for you and getting that initial foot in the door. Not so easy in Australia at the moment.
The hardest part is the work isn't just complicated, but the workload requires a lot of dedication and focus. Things not a lot of people are willing to give up their lives for four years for.