Is this a bad simile?? 'Lightning clashed like broken shards of glass across the ominous darkness.'

It's not a bad simile because as long as it compares two unlike things using "like" or "as", then you should be fine. However there is a difference (at least in my head) of similes that are worded well and worded poorly. Yours wasn't worded poorly, but I think that it could be better. I'm not trying to put you down and you don't have to do what I suggest or anything like that if you don't want to. Just hear me out for a bit.

What I like about this phrase is that it carries quite a bit of emotion. It makes it seem as if the phrase is explaining a jarring thing that is powerful and very noticeable (correct me if I'm wrong). The feel for it is very strong. Your word choice is great!

The only problem that I personally have is that even though the feeling I get from it is strong, it doesn't really give me a strong visual in terms of comparison. I can sorta see what you mean, but I got a bit confused about the "ominous darkness" part. Did you mean that the lightning clashed just like how broken shards of glass would in ominous darkness or did you mean that the lightning clashed in ominous darkness just like how broken shards of glass would? Maybe it's just me being weird, but I sorta found it a bit ambiguous. But I do hope that you meant the second one (lightning in ominous darkness clashing like shards of glass) because that one makes the most sense. 

So the point is to not get too wordy and carried away with your simile that it muddies up the meaning. But if you don't want to change it too much and still want to keep the words you have now (which I personally would do since I love your word choice), try to rearrange your words and/or appropriately  add commas (so many people have trouble with this, even I make a lot of those mistakes especially in informal writing) in order to prevent such ambiguity.

I hope this helps and you get what I mean :)

Thank you :D yes I meant the second one , How about across the ominous darkness of the sky, lightning clashed like broken shards of glass (or something) .I'm not so good with all these literary things since I'm 13 lol but I've been working on a book for a couple of days now.Here's what I've done so far-

There was a storm that night. Thunder  shattered the silence
and across  the
ominous  darkness  of  the  sky
lightning  clashed  like
broken  shards  of
glass . Afraid that my horse would take a fright and
throw me I dismounted from the
once dazzling white beast , that was now
clad with mud , and tucked my
rough archaic trousers into my
tough leather boots . My legs were
as heavy as lead
( which is supposed
to be the heaviest metal ,
according to the books I’ve
read ) ; fortunately , I could see
an inviting little inn
standing
in the gathering
mist , as if it were
a lonely sea siren
beckoning me to come.

 My breathing was troubled
as I hurried
along the road in
a cautious haste ,
grasping my horses
rein in my trembling hand and making
sure a rebellious strand of
my dark brown hair
had not come
loose from its
concealing cap . When I finally reached the solid oak door
I peered in
the window to make
sure it was in fact
an Inn, as the picture on the sign that swung on
its rusty hinges
was too faded to
see. 

To
the  far  end
of  the  room
blazed  a  comforting
fire, with
a  black  fireguard in
the  shape  of
a  phoenix  and
dotted  around  were  tables  and
stools  with  beer,
cards  or  food
either  standing  or
spilt .

There were a few shady looking
men slouched  down fiddling
with  their  drinks
and  women  pacing
the  room  with
pitchers  and  flirts . It
seemed  friendly  enough
and  my whole  body
was  numb  with
the  cold  as
the  harsh  rain
stung  my  skin
and  ran  down
my  cheeks  so could
bare  it  no
more  and I  finally
gave  the
door  a  push ...

I'm thinking of starting again because it seems a bit rushed to me :( .