It depends on whose side you are on. Advertisers and publishers want opposite things from CPM.
For Advertisers: Lower CPM Is Usually Better
If you are buying media, a lower CPM means you are paying less to reach 1,000 people.
If your budget is $1,000, a $5 CPM buys 200,000 impressions, while a $10 CPM buys 100,000 impressions.
That said, the cheapest CPM is not always the best one. A higher CPM can still be worth it if the audience is better or the traffic converts better.
For Publishers: Higher CPM Is Usually Better
If you are selling ad inventory, a higher CPM means more revenue for the same impression volume.
With 1 million impressions, a $10 CPM generates $10,000 in revenue, while a $5 CPM generates $5,000.
But there is still a tradeoff. If rates go too high, buyers may spend elsewhere and total revenue can fall.
So the better CPM depends on your role. Buyers usually want lower costs. Sellers usually want higher rates. In both cases, the useful question is whether the price still makes sense for the quality you are getting.
Do not judge CPM in isolation. Look at audience quality, campaign goal, and what happens after the impression.