Is it educationally worth it to read Tristan and Isolde?

Since the story is among the first romances ever written, it has had a considerable influence on Western literature and culture - so yes, reading it could help to inform your understanding and appreciation of other Western works of art.

As a story of adultery, it comes from a time when marriages were mostly arranged, when often the woman was an unwilling pawn in a political (or financial) arrangement between the parents. The lovers' use of a "potion" could be interpreted as a metaphor on many levels.

It could be construed as a morality tale of just how tragic and lethal jealousy can be - although there are so many variants of the tale, that it would depend on which version you read.

I suppose it depends on what kind of education you want.
For me, as a writer, Tristan and Iseult (or Isolde) is essential foundation reading.

Since there are more books in the world than anyone could read in a lifetime, we each have to choose what we read according to our preferences and goals.

What about a book attracts you? 
Nonfiction is easy to understand;
we learn facts, whether it's 500 years of revolutions in Western societies
or a proof of the plasticity of the mind.

If your bliss is a good novel, then possibly your goal is relaxation, an alternate dreamworld, a form of entertainment, a way to escape from the realities of daily stresses. Or it maight be a way to empathise with the deeper psychological truths of people in situations and cultures other than your own. In a way, it's sort of therapeutic, though not actually a serious therapy.

If you look up "Bibliotherapy.com", you can find a branch close to you.
For a fee, you can consult with a specialist about your issues and/or gaols
and receive a lifetime's list of the best books you could read for your needs.

I can't afford the $125 at the moment - but I'm saving for it.
For me, it will count as an excellent investment in my education.