Is it ok to use medoo framework (http://medoo.in/) when devloping new plugin. or should I use WP_Query only?
medoo and other similar libraries are generic, that is they are aimed at querying some database in general and the weight of figuring out necessary queries are on you as developer.
WP_Query and other WP APIs that perform requests to database are much more specific. They already contain much of that logic, which allows you to express what you need in API calls, rather than SQL query or abstraction close to it. Related code in core implements enormous amount of SQL query generation with literally years of logic and performance optimizations.
Overall for any WP development you have to use its APIs as much as possible, or you will have to waste incredible amount of time reinventing what they do.
Though bringing your own database abstraction can certainly be beneficial for some use cases. For example if you plan to develop extension which will add and interact with its own database tables, rather than native WP data structures.