I asked my mum to make me French onion soup and i hate it?

No, that might hurt her feelings. Just say that you're too full to eat anymore. 

No, just don't ask for it again.

She forgot the Sherry.

Of course you should tell her. Why hesitate to share your opinion with her about it? Its' not like your saying something negative about her just because you didn't like the soup.It will be an opportunity for conversation with each other as well.

Say it nicely, something like "I don't think my taste buds have matured yet to appreciate the soup. Thank you for doing that for me. You're the best."

Onion soup takes quite a while to make properly. You don't Just sweat the onions, you carmelize them-that can take over an hour. And it often includes sherry (I use cream sherry as the wife likes it). And a Lot of it is presentation-the cheese melted over the bowl and the chunk of Toasted bread inside. 

No.

And I suggest you make it yourself.  Use a real french recipe and caramelize the holy bejesus out of those onions.  Takes at least 45 mnutes to 1.5 hours.

I was not so much a fan of french onion soup til my 50s.  Turns out I had never had it made correctly.  There is no way to do a 'hurry up' version anything like the real thing.  I was working with a local cooking school and we would do French Onion soup as one of the things we cooked in our knife skills class - teaching 20 people how to cut onions properly produces A LOT of cut up onions.  The schools founder, who is an amazing chef, would be on  the hot side of the kitchen doing the soups with the veges we cut up during class.  Her soup was so amazing I went home and made it over and over.

Another soup I make that has a caramelized onion base is my mushroom soup.  I made that a couple weeks ago and used four  very large  onions.  I was  making a couple other sauces at the same time so had the heat a little lower than if I'm standing over the pot (which you still have to do toward the end) it took 2 hours to caramelize the onions.  I did end up with a HUGE batch of soup, had four pounds of crimini, 2 pounds of portobello and 2 oz of rehydrated dried porcinis in it.  Of course butter and thyme and tarragon and chicken stock and salt and pepper and cream.  Took some to a couple of my favorite neighbors, had soup for a couple days and plenty went into the freezer (without the cream) for future meals.

It takes a lot of time and patience to make a GOOD French Onion soup.  Just getting the onions sliced is daunting and to get them properly caramelized can take 45 minutes to an hour. 

So yes,  tell her you did not care for it.  You can tell her why if you wish.. Thank her also. 

You would not like the best onion soup in the world.  I never eat it.