The data within the database has functionals relationships. For example, user_meta and user. In that regard, it would be considered relational data.
On the other hand, if you mean, that referential integrity is strictly enforced, no it is not. This is mostly a product of MyISAM which is faster but does not support foreign key checks. On the whole, with projects such as WordPress, a simple index of fields used in WHERE clauses more than suffice.
This is often the same as most projects that use MySQL – the table relationships are implied through naming but enforced at the logic layer, not the data layer. This is because only WordPress is expected to read the database and nothing else. Therefore there is no reason to put data and key services into action (which is the design rationale behind MyISAM (as far as I understand).
If you were to dig intot the core of WordPress, you would find WHERE clauses and even joins which expect there to be a relationship between data in one table and data in another. Comments would not work if the individual comments could not be tracked back to the post they were made on. Which is why comments and posts are considered related.
TL;DR: Yes but the key relationships are not enforced due to design considerations.
As to if WordPress covers the requirements of, say, third normal form, that’s a whole other debate.