I would not be so naïve as to say that grandmothers are the only 
subjects of bad poetry. I would only point out some of the narrowness and shallowness of the genre. How about this classic opener:
Everything my grandma does
Is something special, made with love.
There is even a subset of poems on the subject of grandmothers’ aprons. Full disclosure here: I had an adored grandmother who actually sewed aprons both plain and fancy, that constituted an important part of her meager retirement income. Decades later, we found that several of the fancier examples had been carefully preserved.
But you don’t see me writing a poem about it, do you?
The poems you do see are breathtaking in their singsong blandness. This one was found (where else) in an online grandma apron poetry section:
When I would visit Grandma, I was always very blessed
By the apron that she wore, and the love that she expressed.
And on for several stanzas.
Clearly, for this genre, grandma equals love and giving and sweets and an old timey sensibility. What could ever be that good again? When are we ever, in real adult life, the recipients of food, devotion, and the warmth coming from the old wood stove? The answer, my friends, is never.
So here is my contribution to the grandma poetry canon:
Now that I am Grandma, I
Set the kids on stools so high.
They mix their snack, I mix my drink,
We dump the dishes in the kitchen sink.