Well, Edna, Why WOULD she make a good Jedi?
I see I am talking to a Harry Potter fan here. After all, we know who Remus Lupin is, do we not?!
However, to answer *your* question, I must *first* define what a great Jedi Knight is.
A fine Jedi Knight is someone epitomized by the likes of Qui-Gon Jinn. Think Qui-Gon, think fine Jedi Knight. There's your Short Answer to the (complicated) Question.
Here is an abbreviated version of the Long Answer:
A fine Jedi Knight is:
Since high Wisdom (in the D&D sense) is the Prime Requisite (in my book) of the Jedi Knight, a fine Jedi Knight probably has very high wisdom.
This means, more exactly, a high level of self discipline, self control, inner peace and quietude (something *very* hard to attain, obviously), a high level of judgement and the ability to act decisively on those judgements, an ability to make calm and thought out judgements, humility, open mindedness, a non-judgemental approach, a lack of elitism or elitist attitudes, a lack of arrogance, a lack of righteousness, a strong sense of ethics and understanding of the importance of rules without being supercillious, and other things.
Qui-Gon Jinn has these qualities. So does Luna Lovegood.
A fine Jedi Knight has a high degree of the stat known as Observation (sometimes, the 8th stat in D&D, after Comeliness.)
This means, an ability to focus and concentrate on one's surroundings when one tries, an ability to maintain focus and concentration, an ability to absorb vast amounts of information from observation, that esoteric thing called insight in large quantities, intuition in healthy doses, the ability to see and hear and listen, and perceive things that others would miss, and draw conclusions from information that others would not make.
In a sense, then, Observation and Wisdom are linked. You could consider Observation to be a subset of Wisdom.
Qui-Gon Jinn had a very high score in Observation. So did Luna Lovegood (a point missed by most people in the books, until they found this out the hard way, Quidditch commentary not withstanding.)
It should be noted that a fine Jedi should be a *caring* individual. A *kindly* individual. A *considerate* individual. Decent, forthright, ethical, trustworthy, the kind of person you can count on to stand by you, and who you can let guard your back.
I would even go so far as to say the model Jedi is a gentle, loving person, a peacemaker through dialouge, a warrior only as a last resort.
Qui-Gon was like that. And, incidentally, so was Luna Lovegood.
Intelligence is the second prime requisite of the Jedi Knight, in my opinion.
What is intelligence? In this case, just go with the general concept. The ability to learn, memorize, deduce (Sherlock Holmes, anyone?), all the things that made Hermione Granger so outstanding.
But the greatest and most relevant thing here, was the capacity to weld Intelligence, Wisdom, and Observation together, to create something greater yet. 1+1+1 = 15.
Qui-Gon Jinn could do this. And so could Luna Lovegood.
The final Prime Requisite is Charisma, and here things become more dicey.
Charisma is attractiveness, in an overall sense. Is this person likeable? Got a great personality? A neat person? Attractive? Romantic? Just ... someone who impresses and you just think that person is alright?
Well, Qui-Gon was like this (most people I have ever talked to, seem to think so ...)
Luna? Luna had a much harder time here.
Luna was compromised by the fact that she refused to prevaricate. She told the truth. Just the truth, as she saw it. Sometimes, this did great things. Sometimes, the truth went over like a lead balloon. And sometimes, the truth went over people's heads, and they would gape in incomprehension.
For example, Luna Lovegood would not hesitate to call out the Jedi Council of the Old Republic for the incompetents and hypocrites they were. Not out of malice, but just because it was the truth. She would go on to explain to them (in detail) the necessity of training Anakin, and their foolishness in falling to the Dark Side in giving in to fear, and a lot of other rather painful things (for them) to hear.
And when they became outraged by this, Luna would matter-of-factly, calmly, even gently point out that giving in to Anger was another failing, another way to the Dark Side, then ask them why they insisted on such foolishness?
In short, Luna would go over with the Jedi Masters like ... well ... I think they might prefer a conversation with Darth Vader instead.
Luna also had a particularity: she took the attitude that if a reliable source told her that something was real, it was real (unless someone proved otherwise.)
For example, Narkles were real. So were other things far more impossibly fantastic (that is, fantastically impossible to believe.)
This caused Luna quite a lot of trouble with her peers, but it remained her way of thinking for the rest of her life.
Yet ... in the end, Harry and his friends - as antipathic to Luna as everyone else at the beginning, especially after bad first impressions - eventually became extremely close friends with her, and in the end they were willing to trust her with their very lives.
So where did Luna stand, with Charisma? Somewhere difficult to define, somewhere Enigmatic.
Then again, Qui-Gon was willing to be unpopular when he felt it necessary, even defying the Jedi Council itself repeatedly (and being punished for it, since they played politics and refused to promote him to the Council.)
Luna was not interested in popularity contests anymore than Qui-Gon. Luna was interested in her particular subjects of interest, and in ... being the decent, caring person that she was.
Strength? Dexterity? Constitution?
Unneeded for a Jedi. They used the Force to substitute for all three. With the Force, they could employ strength, dexterity, and endurance far beyond what an olympic champion could hope for.
So Luna's slight, frail build is irrelevant here.
General health?
Both Qui-Gon Jinn and Luna Lovegood were in top health. Qui-Gon, because of his general duties. Luna, because of the demands of Hogwarts.
Does this help? There's a version, as it were, of the Long Answer.