More active masternodes does mean less rewards for each one--each masternode, that is. Each block has both a staking and a masternode reward--currently 2.8 PHR for staking and 4.2 PHR for masternodes. That makes each reward pool separate and distinct, so when a masternode is added, each existing masternode will be rewarded slightly less frequently.

However it also means that 10,000 more PHR have been locked as masternode collateral, which means that amount is no longer available to be staked, so staking ROI will tend to go up as the masternode population goes up.