Without Lyanna Mormont, Jon Snow would likely not have been crowned King of the North in the Season 6 finale.
If Ser Davos hadn’t managed to persuade Lyanna, Jon may not have had any forces of House Mormont at his side in the Battle of the Bastards.
Sure, Lyanna only gave them 62 men from Bear Island for battle, but it was a start, and her allegiance was ultimately more valuable than any fighters she might have mustered.
The siblings (or cousins) were surprised to discover that Lyanna’s youth didn’t stop her from being a strong leader, and it quickly became clear that her support for House Stark did not automatically extend to a bastard named Snow and a lady married to a Bolton.