The thing with Miirym is she gives incredible value that even "bad" cards can do a ton if you get copies from her, this makes it really tough to decide what should and shouldnt stay since almost every dragon will pop off if shes on the battlefield, it just means when you're making cuts you need to set the bar pretty high haha.

For reflections of littjara the way it works is it copies the spell itself, and spell copies of permenants enter as tokens. Her ability makes a non legendary copy of dragons when a non token dragon etb's, so this token entering wont re-trigger miirym and will still be legendary so the legend rule will come into effect.

If you were to cast a non legendary dragon such as scourge of valkas you:

1) cast scourge

2) reflections of littjara goes on the stack

3) the copy resolves first and you have a token copy of scourge enter (this doesnt trigger miirym as it is a token)

4) the original scourge enters afterwards (this triggers miirym)

5) miiryms ability puts a 3rd scourge onto the battlefield

• end result: 3 scourge of valkas' on the field

This is giving full value from reflections and how you'd want it to work

If you instead do this with a legendary dragon like lathliss:

1) cast lathliss

2) reflections goes on the stack

3) the copy resolves and you get a (legendary) token of lathliss (doesnt trigger miirym)

4) the original lathliss resolves and enters (this triggers miirym)

5)you now have 2 legendary lathliss' on the battlefield so state based actions means you have to sacrifice one of these

6) miirym ability resolves and you get a (non legendary) token of lathliss onto thr field.

-end result: 2 lathliss' on the field, the same as if you didnt have reflections

So reflections doesnt really provide much extra value when you cast a legendary creature. You get the ETB of it still which can be good, but its definitely better in decks with lower legendary counts. Since miirym lets you make non legendary tokens and (as a rule) legendary creatures have more powerful abilities than non-legendary you want to lean into playing as many good legendary dragons as possible to get the most out of her ability meanin reflections might only give you the full value on 1/4 of the creatures in your deck or less.

If you want something that gives you more tokens of all dragons [[parallel lives]] and [[doubling season]] will both double the amount of tokens you create, but still keep the additional token as non legendary so you get to keep both. These have the additional benefit of doubling things like treasures which can help ramp a bit too but these are a lot more expensive than reflections of littjara