JOEL - Even though it is the lowest kingdom of heaven, the Telestial kingdom is still a degree of glory "which surpasses all understanding;" as described in D&C 76:89.
Joseph Smith wrote the following in a poem about D&C 76:

"And thus I beheld, in the vision of heav'n,
The telestial glory, dominion and bliss,
Surpassing the great understanding of men,--
Unknown, save reveal'd, in a world vain as this."

Even though the conditions in the Telestial world are so glorious that they surpass our understanding, the idea that one would commit suicide if he caught a glimpse of what the Telestial Kingdom is like is at best speculation and personal opinion.
This idea probably evolved from an entry in the diary of Charles Lowell Walker quoting what he heard Wilford Woodruff say who was quoting what he heard Joseph Smith say:

“If the people knew what was behind the veil, they would try by every means to commit suicide that they might get there. But the Lord in his wisdom … implanted the fear of death in every person that they might cling to life and thus accomplish the designs of their Creator.” (Diary of Charles Lowell Walker, ed. by A. Karl Larson and Katherine M. Larson [Logan, Ut.: Utah State University Press, 1980], vol. 1, pp. 465-66.)

Note that this third hand report says “beyond the veil” not necessarily the Telestial Kingdom. Other than this there is no known quote on this that can with certainty be attributed to Joseph Smith.