Trout is not funnyNah, it's because she's quite ordinary/not particularly interesting, yet somehow has a weekly ~8-minute spot on one of the station's most popular shows, plus other lengthy moments when she calls other programs. You said it yourself: "She's no different than all the other regulars who call in, some also every day/night. "
She's only on there these days because it gives Hutchy the opportunity to seem like a top bloke with everyone hearing how "nice" he is to her. He obviously thinks it's good for his image.
Someone could argue that Trout From Woodend (or a bunch of other regular callers) should get a similar spot because he's funnier and knows more about sport. What's the difference? The difference is that it's much better PR for Hutchy to seem caring to a sweet old duck.
She was mildly amusing when she first started calling in to shoot her mouth off at Malthouse, et al, but the novelty wore off ages ago. She's a case of small doses being fine, but bigger doses being crap, and no amount of Hutchy's fake laughing at almost every word she utters will make change that