Ways you can improve your code:

• The body of the if should be on a new line to the if. Which makes it much more readable.

• Dictionary comprehensions are easier to read than dict(map()). Incase you didn't know Pythons BDFL Guido Van Rossum has had plans to remove lambda, reduce, filter and map from Python. And so I can't encourage the use of map or lambda, even if they're still in Python3.

• Your comprehensions are hard to read. Don't put them all on one line! Spread them out.
  Take:

  art = '\n'.join(['\n'.join([' '.join([figures[x][i] for x in t.lower() if x in figures]) for i in range(height)]) for t in text.split('\n')])
  

  And:

   art = '\n'.join([
       '\n'.join([
           ' '.join([
               figures[x][i]
               for x in t.lower()
               if x in figures
           ])
           for i in range(height)
       ])
       for t in text.split('\n')
   ])
  

  I can read the second much better than the first. I also don't know anyone that would object to this.

• I find assigning to a variable to then return it quite redundant.

Ways you could improve your design:

• I find the creation of figures to be quite hard to read. What you're doing is making a dictionary of data in groups/chunks of height. Instead of what you're doing now, you could instead use the itertools recipe for grouper. And zip the result with the keys.

• Your could add a check to see if the data is divisible by the height. This is a basic check to see if the data is actually correct. As when I was checking my changes, I didn't use a raw multi-line string. Which this check would warn about.

• You can merge the outer two '\n'.join comprehensions into one. This is kind of hard to explain, but since you're making a list of list of strings. And you're performing the '\n'.join on both the list of string, and the list of list of strings. To get a string, you can just do it on a list of strings.

This can result in something like:

from itertools import zip_longest

def grouper(iterable, n, fillvalue=None):
    "Collect data into fixed-length chunks or blocks"
    # grouper('ABCDEFG', 3, 'x') --> ABC DEF Gxx"
    args = [iter(iterable)] * n
    return zip_longest(*args, fillvalue=fillvalue)

def asciiart(text, conf=default_conf):
    if isinstance(conf, str):
        conf = conf.split('\n')
    height, keys, data = int(conf[0]), conf[1], conf[2:]
    if len(data) % height:
        raise ValueError("Config's data is not divisable by it's height.")
    figures = dict(zip(keys, grouper(data, height)))
    return '\n'.join([
        ' '.join([
            figures[x][i]
            for x in t.lower()
            if x in figures
        ])
        for i in range(height)
        for t in text.split('\n')
    ])