Market cap is a piss-poor metric to compare in isolation.
Hey guys I just made up "DumbshitCoin" and it has 40 million possible units priced at a dollar each!
You are not completely wrong, there may be better metrics than market cap. But what you say in your second sentence is true for smaller altcoins, but the top 10 altcoins have already a pretty mature market where this kind of manipulation is no longer possible.
Still, as I said before, the main driver in the altcoin market is not an
already occurring mass adoption, but speculation on a
future mass adoption - partly caused by Bitcoin's "scalability stalemate".