Not necessarily. The negotiated link rate depends on both connected ports and the cable path; some devices are designed with a 100 Mbps interface.
Why this happens
A network link report describes the connection between two pieces of equipment, not the speed of the whole internet service. A television or adapter may have a lower maximum Ethernet rate than the router. In that case, seeing 100 Mbps can be normal for the hardware. A link unexpectedly falling below the capability of both ports can be a different problem, but the number should be compared with the device specifications before buying a replacement cable or assuming every connection must negotiate at gigabit speed.
Read the specifications for the actual TV, box, adapter and router port. Then compare the measured internet throughput and playback result rather than treating the negotiated rate as a quality score. Google Nest’s troubleshooting guidance, for example, discusses Ethernet speed limits and the importance of the connected path. If the link rate changes when a known-good supported cable or port is used, keep that result. Do not remove a stable wired connection simply because Wi-Fi advertises a larger theoretical number; practical delivery and the video’s needs are the relevant comparison.
A useful way to work through it
Compare the link rate with the specifications of both connected ports.
Test a known-good cable or supported port only when the observed rate is unexpected.
Judge the actual playback result as well as the negotiated number.