人魚が卵を産むことはもっともらしいと思います。魚が魚であり、ほとんどの魚が多数の小さな卵を産むので、体の底部があるため、人魚がそうするのは正常です。ただし、人魚が別の方法で繁殖することが可能です。
I think that it is plausible for Mermaids to lay eggs. They have bottom part of their body to be as fish and most fishes lay a large number of small eggs, therefore is normal for Mermaids to do so. However it is possible for the Mermaids to reproduction in another way.