When I try converting a matrix to a data frame, it works for me:

 > x <- matrix(1:6,ncol=2,dimnames=list(LETTERS[1:3],letters[24:25]))

 > data.frame(x)

   x y

A 1 4

B 2 5

C 3 6

 > str(data.frame(x))

`data.frame':   3 obs. of  2 variables:

  $ x: int  1 2 3

  $ y: int  4 5 6

 >

You can also use as.data.frame() to convert a matrix to a data.frame

(but note that if colnames are missing form the matrix, as.data.frame()

  constructs different colnames than does data.frame().

=========================================

> data <- c(0.1, 0.2, 0.3, 0.3, 0.4, 0.5)

> dimnames <- list(time=c(0, 0.5, 1), name=c("C_0", "C_1"))

> mat <- matrix(data, ncol=2, nrow=3, dimnames=dimnames)

> as.data.frame(as.table(mat))

  time name Freq

1    0  C_0  0.1

2  0.5  C_0  0.2

3    1  C_0  0.3

4    0  C_1  0.3

5  0.5  C_1  0.4

6    1  C_1  0.5

=========================================
REF:
https://*.com/questions/15885111/create-data-frame-from-a-matrix-in-r
https://stat.ethz.ch/pipermail/r-help/2006-January/085978.html