Is Jesus the true God mentioned in 1John 5:20?

No.

This passage is a poor translation of the KJV. As written Jesus is clearly spoken as the “Son of God” but the ending part of the passage can, understandably, be somewhat confusing because it is what we would call an Ellipsis statement.

This following is actually a more concise rendering of 1John 5:20.

The Book of Yahweh

“And we know that the Son of Yahweh has come, and has given us an understanding, in order that we may know Him Who is true, and we are in him who is true; and we are in His Son, Yahshua Messiah. Yahweh is the true Father, and eternal life.”