happy birthday OP! hope you've had a good day otherwise... And no, don't think you are wrong to feel hurt by the lack of thought that your mum has put into your present - I'm sure you would have had a lot more pleasure had she sent you even half the value in money or gift vouchers!
I'd ring your mum and tell her that the trousers are the wrong size and a bit mumsy for you so please could you have the receipt so you can change them (omitting to mention that you were with her when she bought them!) for the right size and something that you like and see how she reacts... If she blusters that she has lost the receipt then you can say 'oh never mind, they are your size aren't they - I'll send them back to you so you can wear them' and see what she says. Depending on what she says (and if she volunteers to send something else in their place!) you can either give them straight back or wrap them up for her for her birthday/christmas (whichever comes first) - and she can hardly complain because you will have told her you are going to do that.
I'd also tell her that you think she has been done by the people that sold her the gift set as several of the main items have been taken out of it (is it one of those sets that has a little list on it somewhere of all the bits in it with their ingredients on, or has she taken that out to pre-empt that?) If there isn't anything on it that says what the contents should be, then if she challenges you, say that you looked it up online to double check the ingredients and the only set they did that had the ingredients in that she sent should also have had the other stuff in too.
And depending on whether or not you like the stuff in the gift set that she has left in it for you - tell her that it's the same as the stuff in the last set she gave you for christmas and it made your skin go funny but never mind, you know she likes elemis so you'll swap it with her when you next see her for something that you like from her regifting present box (does she have one of these? sounds like she does...)
and then sit back and enjoy the show as your birthday present!
if all else fails, give her something that you like that you think she won't for her next present and hopefully she'll give it back to you for your next present - at least you'll get something you like! 