For those of us watching EE from the beginning or thereabouts then Kathy was definitely a significant character throughout most of the 80s and 90s....in the 80s particularly as Mrs Beale of course, before her husband and Ian's Dad (Pete Beale) passed away - and before she was foolish enough to then become 'a Mitchell' in the 90s. For me she came most to prominence in the late 80s after the James Willmott-Brown rape ordeal - something which was ground breaking for a soap to cover/explore back when the episode aired in 1988.
Re the questions concerning Kathy's past role in EE, then I'm guessing the majority of posters weren't even born (or certainly weren't "with EE" from it's mid 80s beginning!) and therefore to that generation Kathy Beale would just be a name referred to occasionally by Phil, Ian and of course Ben in relatively recent years without bearing any genuine significance.