Coming up with an alternative to the word “lame” seems like the wrong way go about this. To you, a lame thing is a thing you don’t like. That’s what it means to me, I’d wager that’s what it means to a lot of people. Saying it is a knee-jerk reaction. Any word you pick to replace it will have the same negative connotation. Instead of picking a short word to use to describe a thing you don’t like, stop and think about why you don’t like the thing and use that reason as your statement of disapproval. Or, if you’re just saying you don’t like a thing, maybe that sentiment doesn’t need to be expressed.