Here's a quick (and dirty?) trick to create a tag cloud in Pelican without plugins and minimal amount of code.

Define a jinja filter in to convert a tag's article count to a CSS font-size value. For example like this:

import math
    'count_to_font_size': lambda c: '{p:.1f}%'.format(p=100 + 25 * math.log(c, 2)),

(Play with the parameters of that mathematical expression to tweak the size mapping to your liking.)

In the tags.html template you can now use this filter to inject the font sizes as inline CSS. For example, in a typical implementation that renders the tags listing as an item list:

 <ul class="tags">
     {%- for tag, articles in tags|sort %}
        <li style="font-size: {{ articles|count|count_to_font_size }};">
            <a href="{{ SITEURL }}/{{ tag.url }}">{{ tag }}</a>

For a pretty tag cloud, you may also need some CSS rules on the corresponding list elements (if not already), in essence something like this:

ul.tags { list-style-type: none; }
ul.tags li { display: inline-block; }

and probably some more margin and padding tweaking.