Not necessarily. A low market cap coin can certinaly be a hidden gem that no-one else knows about and will shoot up in value. But there is also a huge risk. One problem is that it is easy for pump and dump groups to manipulate the price of low cap coins, so beware sudden price moves. Another is that low transaction volume may give an artificial picture of what the coin is worth. The biggest risk is that the coin could just fade to $0.
Low cap is where the biggest % returns are, but also where the biggest risk is.
I think a good strategy is to have some high cap, some mid cap and some low cap coins.