Is the maintainer of a database a good or bad reviewer in principle?
It is impossible to say whether they would be good or not, but it is fine to suggest them. It will be the editor that chooses, in any case. And they are unlikely to be the only reviewer. Don't worry about it. If you think they have the required skill it is fine to suggest. You don't need to make a judgement.
The editor, being human, will use their own judgement. But a reviewer, promoting their own work and giving unwarranted positive reviews will probably be easy for the editor to see.