I think there are two ways of being polite, one is that fake-politeness you mention, the other is more of an avoidance of proactively hurting people’s feelings.
As an example let’s say you think gender reveal parties are stupid. Your friend invites you to their gender reveal party for their baby. Declining the invitation with a fake excuse is the first kind of polite. Just declining by saying you don’t want to go is the second. Proactively offering your opinion that gender reveal parties are stupid in general and your friend shouldn’t have one even though they didn’t ask and probably already scheduled it would be unnecessarily hurtful. On the other hand if they asked you what you think of gender reveal parties instead of inviting you I would not see it as rude to respond honestly. Also, if you decline politely and they keep probing deeper for reasons you are under no obligation to make up some fake reason.