Feelings of sinfulness and inadequacy are natural emotions when faced with the perfection of God’s throne. Isaiah also may have felt insecure about his role as a prophet to the nations. That the Lord has power to cleanse his sins and ‘make weak things become strong’ (Ether 2:27) is evident in the ensuing verses.
Robert S. Wood
"President Spencer W. Kimball warned of vulgarity of speech and expression and particularly counseled against speaking of sex glibly, which he associated with immodesty. ’Lewd talk and jokes,’ he said, ‘constitute another danger which lurks seeking as its prey any who will entertain it as the first step to dirtying the mind and thus the soul’ (The Miracle of Forgiveness, [1969], 228)…
"I’ve been stuck by the fact that when Isaiah received his charge from the Lord, he bemoaned that he was ‘a man of unclean lips’ (Isa. 6:5). This sin too had to be purged from Isaiah if he was to bear the word of the Lord….
“We need to eliminate from our conversations the immodest and the lewd, the violent and the threatening, the demeaning and the false.” (Ensign, Nov. 1999, 83 as taken from Commentaries on Isaiah in the Book of Mormon, ed. by K. Douglas Bassett, [American Fork, UT: Covenant Publishing Co., 2003], 122)