My poetry was called simplistic? ?

I don’t think it’s negative. The sentence use “although”, means the person like simplistic poems. The sentence means although this poem is not in the way they used to like, they like it.

He's saying he likes your poem, even though it isn't as simple as the others you've written.  He enjoyed your simple poems.  This one was different, but it still his favorite.  

I think it wasn`t intended to be taken as a put-down of your other work;
I read it as meaning that although your others were well-liked for their simple 
(pure/ unembroidered, engaging) merit
this outdid them as the favourite.

My guess is that they could have used other expressions that put a different spin on it. They could have called it 'unpretentious' or 'direct' or uncomplicated or 'clear communication of ideas'. Simple is often best if you really intend to communicate. Your poetry communicates its intended message. It does not seek to make you look clever or superior. It's about the ideas, not your ego. We can all learn from constructive criticism so I wouldn't ignore it but I would wonder if the criticism was intended to make the critic feel superior rather than to be helpful.

Yes.   For example, most of the poems by teenagers which are submitted to this site for general criticism, contain a 'message' of some  sort.   "war is bad, love is good'.  'Dumping waste about the place is not nice', ' I love my  mother/kitten/football'  and so on. 

You will have to consider what was meant as “the others”. 

If by the others, they meant compared to your other poems, then you are right that they think your other poems are simplistic. But I would think that at least some of them would have written: “... not as simplistic as YOUR other poems”.

But, I am more inclined to think that they meant comparing your poem to other people’s poem of the same genre. In this case, their comments would be a compliment.

It sounds like someone who thinks 'simplistic' is the grown up word for 'simple' - it isn't.

It depends on whether it was meant as an insult or a compliment.

called so by whom? A teacher? Don't worry about it. That teacher isn't going to win any prises himself/herself and is likely just bitter due to their failure.
Simplistic implies your writing is ordinary and trite. Try using a thesaurus.

I think you have a habit of seeing the black dot and not the vast  white background surrounding the black dot