Is Lyanna really as terrible as some people portray her as?

no, not even a little bit.

the absolute most important thing about lyanna is that when she dies she is only 16. i am someone who works with kids - i work in a library so i spend most of my days cleaning up after tweens and asking teenagers to please stop doing dumb shit- and the first thing anyone who has ever worked with kids and especially teenagers is that they may look like adults but they are NOT. they don’t understand boundaries, they have next to zero impulse control, and every bad thing that happens feels like the worst thing ever because it very likely IS the worst thing they’ve ever experienced bc they have not been alive that long!

and this goes for every single teen & tween character in this series, not just lyanna! shit, i am someone who feels an immense amount of sympathy for joffrey! on one side he’s got his mother telling him he can do anything he wants with no repercussions and on the other he’s got his father hitting him so hard that stannis thought joffrey was going to die. and then he is given unchecked power and told not to abuse it! EYE cannot even guarantee that i wouldn’t use unchecked power to do shady shit and i am a fully grown adult, not a traumatized, irrationally, and deeply vindictive 13 year old boy.

but honestly the most important thing about lyanna is that we have ZERO CONTEXT for what happens between her and Rhaegar. What we have is

What’s important to note is that our view of her is heavily filtered through the eyes of the men that knew her. Robert loves an idealized version of her that never existed. Barristan never actually knew her. Ned is not only viewing her under 200 layers of guilt and grief, but very obviously does not understand his sister, or why she made the choices she did, and struggles constantly with knowing that he will never know her the way he wishes he could, the way he thought he did. Given the way Meera describes Lyanna, I actually think Howland is our most accurate look at her but even that is buried behind years of grief & a fair amount of hero worship and affection (“that’s my fathers man you’re kicking howled the she-wolf” is a line that makes me WEEP for this exact reason; Howland sees Lyanna as his hero above all else!).

All of that to say - we don't even know what Lyanna did that was so terrible! Even if she was a grown woman capable of making rational decisions, we have no idea what her decisions were. She could have been lied to, misled, kidnapped, threatened, just as surely as she could have walked into the situation with open eyes. Even in the show, with a slightly aged up Lyanna - we get, what, just Sam's opinion on Rhaegar and Lyanna being in love because they got hitched? Completely ignoring the fact that we had several women in this series get married not because they were in love or willing but because someone more powerful decided on it and that was that, so there's still no evidence that Lyanna had enough information about the situation to make any sort of informed, consensual decision.

so no, i do not hold lyanna responsible for anything at all that happened regardless of how it happened because she was not mentally mature enough to understand what the hell was going on. a 15 year old is just not mature enough to think “if i run off with this married man, it’s going to cause a cascade of political issues that could have disastrous consequences.” what she’s probably thinking is “this man says he can help me and i am fucking miserable and no one else will listen.” it’s why we don’t throw 15 year olds who run away to meet up with old dudes they met online in jail when they’re caught (or theoretically why we don’t punish them at any rate). There is one person and one person only who is responsible for the massive fuck up that is the Elia-Rhaenys-Aegon-Lyanna-Jon mess and that is RHAEGAR, the person with the most amount of power who used it in the dumbest way imaginable and got himself, most of his heirs, his wife, and his teenaged mistress killed. The only other people responsible are the Kingsguard who kept Lyanna under lock and key while she lay dying and pleading for her brother to come save her.