An regular input to XSLT must always be XML, so the stylesheet below assumes as input:
<?xml version="1.0" encoding="UTF-8"?>
<string>[Apple,Pie,Pizza]</string>
Stylesheet (XSLT 2.0)
<?xml version="1.0" encoding="UTF-8" ?>
<xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output omit-xml-declaration="yes" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:template match="string">
<root>
<xsl:for-each select="tokenize(substring(.,2, string-length()-2),',')">
<xsl:element name="{.}"/>
</xsl:for-each>
</root>
</xsl:template>
</xsl:stylesheet>
Output
<root>
<Apple/>
<Pie/>
<Pizza/>
</root>
In case you can only use XSLT 1.0, a recursive template to achieve the same in XSLT 1.0.
Stylesheet (XSLT 1.0)
<?xml version="1.0" encoding="UTF-8" ?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output omit-xml-declaration="yes" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:template match="string">
<root>
<xsl:call-template name="elementifier">
<xsl:with-param name="str" select="substring(.,2, string-length()-2)"/>
</xsl:call-template>
</root>
</xsl:template>
<xsl:template name="elementifier">
<xsl:param name="str"/>
<xsl:choose>
<xsl:when test="$str">
<xsl:variable name="element-name">
<xsl:choose>
<xsl:when test="contains($str,',')">
<xsl:value-of select="substring-before($str,',')"/>
</xsl:when>
<xsl:otherwise>
<xsl:value-of select="$str"/>
</xsl:otherwise>
</xsl:choose>
</xsl:variable>
<xsl:element name="{$element-name}"/>
<xsl:call-template name="elementifier">
<xsl:with-param name="str" select="substring-after($str,',')"/>
</xsl:call-template>
</xsl:when>
<xsl:otherwise/>
</xsl:choose>
</xsl:template>
</xsl:stylesheet>