Is the following statement a good thesis for Katherine Mansfield's story "Miss Brill"?This is my thesis: Miss Brill, Katherine Mansfield...

Sadly, this is not a good thesis as it stands for several
reasons. The first is that it is not written in correct
English grammar. Therefore, it is not understandable.

The
second is that (even allowing for corrected English
grammar) it is not clear whether you intend to prove that Katherine Mansfield
demonstrates Miss Brill's isolation or whether you intend to prove Miss Brill's
isolation is revealed through her sensitivity, loneliness and vulnerability--or
something else entirely.

The
third reason is that, as already indicated, the
relationship between Miss Brill's isolation and the revelations of someone (presumably
Miss Brill) being sensitive, lonely and vulnerable is not clear.

Based
upon my best ciphering of what you might possibly actually mean, I suggest you
rewrite your these in this
way:

In "Miss
Brill," author Katherine Mansfield demonstrates Miss Brill's isolation through
revelations of Miss Brill as being sensitive, lonely and
vulnerable.

In this thesis,
you will be proving through textual quotations and
inferences that Miss Brill (1) is sensitive, lonely and
vulnerable and that (2) these proofs reveal that she is
isolated. As an aside, when writing your essay, be certain that each sentence contains a
Subject (the acting, responsible Agent of the sentence) and
a Verb (what the Agent precipitates or
is):

The
author demonstrates something.
The character is something.
The
isolation proves something.
The sensitivity is revealed by
something.

Be sure
each part of a complicated sentence is properly organized
alone, as in the above illustrations, before you try to join
it to another sentence to form an entire complicated thought and
concept.