'Common sense', in this case, means whatever makes sense to you. People don't ordinarily agree on the truth of 'common sense' cases, they are just likely to act belligerently or incredulously towards those who disagree on the matter. When the rules say you should 'use common sense to...', they mean you should make it up according to what seems right to you, and then treat it like objective fact.
Assuming you are the DM, if you think it's reasonable for a character to wear slippers under other footwear, then that's what's reasonable in your game world. If someone else thinks it should be impossible to wear two foot coverings at once, because 'it's just silly', and they're DMing, then that's what's true in their game world. The rule you quote is designed to explicitly move away from a 'magic item slots' system by explicitly telling you to use your own judgement instead. It gives some sparse few examples of the general outline of reasonability, but is quick to point out that that outline is a starting place, not a rule, by giving the example of cloaks as probably more easily layered than the outline makes them out to be.
By asking this question, it seems pretty clear you already think it's potentially reasonable for someone to wear thin slippers under a pair of larger boots. This isn't accurate to typical footwear in the middle ages, but that's only important inasmuchas you care about such things as a matter of authority on your game world. Ultimately, it's up to you, and the rules seems to indicate the system believes the attunement system has you covered, as far as potential balance issues are concerned.