General dullness:
* C'est ennuyeux: This is the most common and straightforward translation for "it's dull."
* C'est fade: This translates to "it's bland" and implies a lack of excitement or flavor.
* C'est monotone: This means "it's monotonous" and suggests a lack of variety or interest.
Specifically referring to a person:
* Il/elle est ennuyeux/ennuyeuse: This translates to "he/she is dull" and refers to a person's personality.
Other options:
* C'est sans intérêt: This means "it's uninteresting."
* C'est sans éclat: This translates to "it's lacking brilliance."
* C'est plat: This means "it's flat" and can be used for something lacking depth or excitement.
The best translation will depend on the specific context and the meaning you want to convey.