I think that if a rav never goes to simchos, it isn’t bad medos because most likely he has a good reason. If a regular person answers that he cannot attend a particular simcha, I don’t think that is bad midos either. If a person says he is attending and then doesn’t, unless he has a very good excuse, it isn’t good midos.
People can’t attend every simcha and as many people pointed out, an invitation, even a personal oral invitation, is not an order.