I want to start speedrunning this game. Is it considered bad by the community when a new speedrunner just mimics routes used by the top runners or I should just make my own route?

Whether you wanna copy existing routes is up to you, that's not considered to be bad or anything, though I highly suggest not copying the WR route because it's really hard for beginners. There are some good tutorials in the "Guides" section, you might wanna check that out. Otherwise you can also use the route used by the runners who got a 2:20 - 2:40 pb. With this route you can get pretty good times and it's not really hard to learn.

watch the Hagginater's beginners tutorial on youtube ;)

I use this http://bkhelper.herokuapp.com/# and use the 100% guide, haggs beginner tutorial is also great

Copying the routes is the smartest move you could possibly do but i`d say its more impressive to use your own routes with getting good times depends on you look at it and not what everyone expects of you.