Yes, it is a positive, but it is impossible to say how much since admissions committees have their own standards and are made up of individuals.
But, if you do well in the research and are able to get a good letter of recommendation from the PI then it will certainly indicate that you are serious about learning "stuff" and practicing the art. While (pure) math research is quite different from engineering research those who can focus and succeed are probably going to continue to succeed.
And certainly it is more positive, even for engineering, than not doing any research at all. And, it might help you decide which field you really want to pursue.
Applied math, on the other hand, might be much more applicable to engineering. And, of course, EE requires a lot of math in any case.