I think your thread title is misleading. I am not sure uncle is awful, at least in this instance.
Your parents were wrong to include the inheritance check in with the wedding card and your mom was wrong in including the note. If my spouse and i opened up a wedding card with a check, we would assume that it was a wedding gift, not an inheritance check. If I had read the note "I know your mom would have wanted you to have this.", I would have thought it was a wedding gift from mom, that she had told brother/sil that she wanted to give me something for my wedding even if she wasn't around, that she still cared for me even if i wasn't the best child in terms of visiting or helping her. It would have raised my hopes and then when I called your mom, only to be told she had not said that, that it was my inheritance check, that my sil had thought it was a nice thing to write, I would be mad and upset and probably not handle myself the best. Which is what uncle did.
Your mom was wrong to mislead him as she did. The inheritance check should have been given separately, not in the wedding card. I think her heart was in the right place, thinking that the newlyweds could probably use the rest of uncle's inheritance as they started their new life. But the check should have been presented differently, separately, from the wedding card and without the note. It was not something his mom would have wanted him to have, it was his due to the provisions of the will. A different note with the check "thought you could use the rest of your inheritance as you two start your new life together" would have been much better and not misleading.
Could/Should uncle have handled it better? Yes. He could/should have waited to send the email for a day or so until he calmed down, chosen his words better. Yet, in some ways i don't blame him for being upset. Your parents started it and misled him/misrepresented the money.
To hold a grudge against him for this is wrong. Both your parents and he were wrong and hopefully they have apologized to each other and gotten past it.