I've got two questions!
can we describe a person by telling that he is spastic when we mean that he is very neurotic as a person.
and the second question is:
in exlamatory structures there is:
1. What + a/an + adjective + noun...!
What a beautiful girl! (and we use this syntax because "girl" is a countable noun)
2. What + adjective + noun...!
What lovely weather! (we use this syntax because "weather" is an uncountable noun)
NO!!! Please don't use spastic to describe anyone at all. It's the term that used to be used to describe people with cerebral palsy but is now considered extremely offensive. There are medical terms which include the word "spastic" but they should only be used when referring to those specific terms and only ever in a medical context.
Your use of "lovely" is correct in both your examples as is your use/non-use of an article.