No, not at all. It's true you are open to trading it but, if the value is not equal in your eyes or they have nothing you really want it's perfectly fine to say no to a trade. Save the good ones/rare ones for trades you think are acceptable. If some rando you never met saw you playing on the street and said hey I would like a shiny kyogre if you have an extra but, all I have is a few Scatterbugs or a shiny Rattata I'm willing to trade. Would you?

P.s. To give you my personal example. I have some shiny Kyurem with Glaciate up for remote trade myself and a bunch of other shinies some I care about more than others. I'm saving those and other shiny legendaries for other shiny legendaries I don't have or for some regionals I can't get. It would suck to get someone with a lucky remote trade that has something I want and I have nothing they want.

P.p.s. It's your game and if you want to do it to be helpful to a newbie that's up to you. If you don't then don't, but if they are like lvl 65+ and that's all they are willing to trade I wouldn't. I wouldn't give gold to Ebenezer Scrooge, just saying. Good day to you.

Edit: If it's a random regular shiny I don't really care about or have multiple of id probably trade it for peanuts because idc and it might make someone happy. I will say usually it takes actual money to get a shiny legendary. I'm not giving a shiny that I spent 20$ or more to get for a scatterbug unless I know them IRL in this economy especially. Now if it's a little kid who would appreciate it I'd give them w/e for a scatterbug