I think "stupid" and "dumb" fall into a category where they're ableist in etymology but in use they are just a common way to say bad or without sense. A lot of derogatory words have that effect, but they create a problem by associating a group of people with being bad (I'm thinking of the homophobic use of the term "gay" in particular). In this case, I think that's rarely the case, I think they've basically shed their associations.

I do sometimes try to avoid them. I'll try to remember what I say instead:

• [X] doesn't make sense
• Silly
• [X] is a bad idea
• I'm not a fan of [X]
• Absurd
• Ridiculous
• [A more specific criticism]
• [X] sucks
• Liberal