if you feel that way then it's better returning it as the 18-55 is one of the worst canon badged lenses and if there's something worse then it really doesn't need to be in your kit bag.
The Canon 18-55mm IS version is generally praised for its image quality for a kit lens. It is substantially better than the original 18-55mm kit lens. However, as you say, a (working) Tamron 17-50mm f/2.8 lens should provide noticeable improvement in almost every area.
Neil, it sounds like you are new to this. I'm not sure if you are aware, but with DSLR lens ownership it becomes almost an obsession for some people when they get a new lens to test it and ensure it is working correctly. Some people like to shoot test charts and the likes but it is true that if you use the lens at maximum zoom close to MFD (Minimum focus distance) this can lead to extreme results where focus (being an analog system) can make errors slightly. As people have said, it is best not to get too worked up on this and spend hours upon end pixel peeping and shooting test charts.
My suggestion would be to go out and use the lens for real life subjects for a day. Make sure you use a high shutter speed and high enough ISO to ensure camera shake is a non issue. Use the lens at wide apetures and take plenty of portrait pictures with shallow DOF. Come back and review the pictures. If they are consistently looking soft then it might be worth giving it to someone with more experience to give it the once over. It is very unlikely to be the body at "fault", although certain camera bodies can be the "problem" in that they are calibrated within spec but at the extreme end of an in tolerance spec whereby the lens might be at the opposite end of said tolerant spec causing issues. This can be checked by using the lens on a demo body in a shop or a friends body.
Anyway, without wishing to complicate matters for you, I would have a play and then check the results back. Make sure you are are far enough away when taking photos. The berry shots of close subjects look like you are close to the MFD (Min focus distance) of the lens. This means you are close to "bottoming out" as it were, where the lens cannot physically focus on an object much closer. Be sure to be more than say 40cm back - I am completely guessing the MFD of that lens @ max 50mm zoom. It may be more or less.
A quick simple test you can do which is very popular just as a quick slap dash test, is to line up some AA batteries at equal distances apart at a 45degree angle to the camera, and take a shot of them. You can google this. The best method is to shoot on a tripod with mirror lock up at a fast shutter speed, high iso, max apeture (i.e. f/2.8) and focus on the middle battery. BAsically everything possible to make sure camera shake is a non issue which can blur the results. You foucs on the middle battery preferably using normal AF phase detect focus, and also compare it to the live view with contrast detect focus. What you should see, is the battery to the left and right of the middle battery focused on drift in and out of the depth of field area equally. This is essentially the same as using a focus test chart. It aims to show whether the lens has issues with front or back focus. This is as it sounds. Some lens can literally drift forwards or backwards from the intended focal point and is the most common reason people receive what they perceive to be a "bad copy" lens. A lot of the time, it is poor testing by the owner or obessive behaviour where there is no problem. This is why I said to just go out and use it in the real world as well.
I hope this helps you.
For what it's worth, the 3rd party lenses from Tamrom and Sigma are much more suseptible to these problems with poor quality control. I have had bad copies before. It does happen. The Tamron 17-50 is very much a key lens this happens on.
At the end of the day, if you are unsure, atleast you have the ability to take it back to the shop and get a replacement since it sound slike your Mum bought it new from a shop? This is so much less stressful than dealing with "dodgey" lenses from second hand purchases!
Good luck. Let us know how you get on.
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