No, you cannot use tha to say what or who someone or something is. Tha is only used to say what it is like, and where, how, when it is. You use tha to describe things and people (and give their whereabouts) and is to classify (define) and identify them. Thus:
- tha thu mòr you are big – describes what you are like
- tha thu gu math you are well – describes how you are
- tha thu ann am pàirc you are in a park – where you are
- tha thu ri Màiri you are with Mary – with whom you are
- is tusa mo bhràthair you are my brother – who you are
- is cù e or is e cù a th’ ann it is a dog – what it is
In general, if the predicate (the thing after is) is a noun phrase (like my brother, a dog, the man who did it, etc.), you must use the copula is. If it is not a noun but a prepositional phrase (in a park, on the table, before midnight, with me…) or an adjective (big, nice, cold…) or an adverb (well, there…) you would use tha (I think sometimes in those cases is is also possible, especially in a few set phrases and poetic language, but not in regular conversational language).