Did you lay the lawn or did you buy it already laid? Is it seeded or turfed?
Can you be a bit more specific, Cat?
I suspect that this is something that will be gone after this autumn, or possibly after next spring. The mushroom compost is full of nascent mushrooms, but they will die when they run out of food, which will be quite soon.
I would suggest aeriating the lawn to improve drainage, and then give it time.
If you're sure they're mushrooms, pick them for breakfast, slice and soften in a little butter or oil and freeze for use in Spag Bol and casseroles etc. I've done it many times.
If you don't want to eat them just brush them off the lawn before mowing, so you don't get a slimy mess.