Tewi: Not at all, in fact even a few people with aphantasia have been successful in creating tulpas that we know of. The core of it is just the feeling of and immersion in another person being present in your mind, and communication (of any form, whether words, visualized interactions, or just pure thought/feeling/emotions) between you.
So, really, any form (or even formlessness) that is comfortable for you to imagine them as is fine. (edited)