This is the sort of thing that Counter
was built for:
from collections import Counter
c = Counter(top_hashtags)
print(c.most_common(5)) # print 5 most common hashtags with counts
To answer your specific question, to sort dictionary keys by value use:
top_ht_dict = {i:top_hashtags.count(i) for i in set(top_hashtags)}
sorted(top_ht_dict, key=top_ht_dict.get, reverse=True)
# ^ most common first