When input data into a low pass filter, are the first several data unusable?
You have to judge yourself if for your purposes you need to get rid of the first few output values. The phenomenon you observe is determined by two factors. The first is the delay of the filter (which is usually frequency dependent). This delay is a consequence of the causality of the filter. For a linear phase FIR filter that delay is independent of frequency and equals $(N-1)/2$ samples, where $N$ is the filter length (as pointed out in a comment by MBaz). The other factor lies in the nature of low pass filters and has nothing to do with a non-ideal (causal) implementation. It is the non-zero rise time that any low pass filter exhibits, even an ideal (non-causal) brick-wall filter. The rise time is proportional to the inverse of the cut-off frequency.

For minimum-phase filters with a very low order (as in your example), the first factor plays only a small role, so the delay you observe is mainly caused by the non-zero rise time of the low pass filter. If you decrease the cut-off frequency you will observe an increase in rise time and vice versa. Note that the input signal that your filter sees is actually a jump from zero (no input signal) to a DC value (plus some small signal), so you pretty much see the system's step response.

In general the response to a change in the input signal is called transient response, and it is not restricted to a change in DC value.