#TOP 1

When selecting values that correspond to Unique fields referenced by constants (OwnerUserId), you're only really selecting one field, meaning this is redundant.

#UVN

_{Unexplained Variable Names}

Don't use names like Q and A, they're confusing at best and unreadable at worst.

DECLARE @Q
DECLARE @A

DECLARE @Questions
DECLARE @Answers

#Yo dawg, heard you like SELECTs

SELECT --...
FROM (
  SELECT TOP 1
    (
      SELECT UpVotes
      From Users
      WHERE Id = @UID
    ) as Upvotes,

SQL is not intended to be used like that.

You don't need to create psuedo tables to select from to store your data.

You can use a base table, and then specify certain fields calling other tables from that.

SELECT
  UpVotes + DownVotes as [Total Votes],
  (SELECT COUNT(*) FROM Posts WHERE OwnerUserId = Users.Id) as [Post Count]
FROM Users

#Formatting

This formatting is a bit weird:

SELECT Score = Upvotes   * @Up +
          Downvotes * @Down +
          Questions * @Q +
          Answers   * @A +
          Comments  * @Comment,

You might want to break these up a little more.

#Naming

These aren't the actual values, they're type weights, so I'd add the word weight to the end of the variable names:

DECLARE @Up      int = 1
DECLARE @Down    int = 2
DECLARE @Q       int = 10
DECLARE @A       int = 5
DECLARE @Comment int = 1

Into:

DECLARE @UpVotesWeight     int = 1
DECLARE @DownVotesWeight   int = 2
DECLARE @QuestionsWeight   int = 10
DECLARE @AnswersWeight     int = 5
DECLARE @CommentsWeight    int = 1

#TOP 1

When selecting values that correspond to Unique fields referenced by constants (OwnerUserId), you're only really selecting one field, meaning this is redundant.

#UVN

_{Unexplained Variable Names}

Don't use names like Q and A, they're confusing at best and unreadable at worst.

DECLARE @Q
DECLARE @A

DECLARE @Questions
DECLARE @Answers

#Yo dawg, heard you like SELECTs

SELECT --...
FROM (
  SELECT TOP 1
    (
      SELECT UpVotes
      From Users
      WHERE Id = @UID
    ) as Upvotes,

SQL is not intended to be used like that.

You don't need to create pseudo tables to select from to store your data.

You can use a base table, and then specify certain fields calling other tables from that.

SELECT
  UpVotes + DownVotes as [Total Votes],
  (SELECT COUNT(*) FROM Posts WHERE OwnerUserId = Users.Id) as [Post Count]
FROM Users

#Formatting

This formatting is a bit weird:

SELECT Score = Upvotes   * @Up +
          Downvotes * @Down +
          Questions * @Q +
          Answers   * @A +
          Comments  * @Comment,

You might want to break these up a little more.

#Naming

These aren't the actual values, they're type weights, so I'd add the word weight to the end of the variable names:

DECLARE @Up      int = 1
DECLARE @Down    int = 2
DECLARE @Q       int = 10
DECLARE @A       int = 5
DECLARE @Comment int = 1

Into:

DECLARE @UpVotesWeight     int = 1
DECLARE @DownVotesWeight   int = 2
DECLARE @QuestionsWeight   int = 10
DECLARE @AnswersWeight     int = 5
DECLARE @CommentsWeight    int = 1