Good question. Rating lens with pixels is obviously technical incorrect, for the simple fact that lenses obviously do not have pixels. However, this is, as you mention, common practice.
Technically, the equivalent rating would be line pairs / mm (lp/mm) which measures how many lines the lens can resolved. This is actually closer to horizontal tv line counts which are commonly used for analog cameras (e.g., 500TVL, 600TVL, etc.).
The problem with using lp/mm is that most lens manufacturers do not disclose this. Instead, roughly converting that to pixels to use the more common, though imprecise term.
Bottom line, a 5MP 'lens' should have a higher lp/mm than a 1.3MP 'lens' but it is unlikely that the detailed specifics will be shared on data sheets.