Yes, you can join the interior designing course after completing the diploma . If you are interested in interior designing you can take up a short-term course in the subject and begin your career in the field.
The knowledge and skills required to succeed as an interior designer are different from those required by an engineer. So its depends on your interest that you want to join this course or not.
If you want you can also pass you 11th and 12 in distance. But it's depend on which institute you want to apply. Because not all institute accept the 10th pass diplomas .
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https://design.careers360.com/articles/interior-designing-courses-in-india