Well, you really don't need to be a good engineer, but the skills required to do anything meaningful in theoretical computer science PhD program are more hardcore and rare than good SW engineers - from what I see, most of the people who can do theoretical CS are already doing it; and most engineering-oriented CS researchers couldn't/wouldn't cross to theoretical CS no matter what.

I mentioned only some subset of PhD applicants, which might not be small. So there are many other types of applicants who apply with different and various reasons.

As for your question, no, I don't think that scientists in all of the branches of CS need to be good engineers. But that is just my personal opinion.