That all depends on what you mean by "stopping mass immigration"
It is perfectly reasonable for us to have informed debates about how much immigration is desirable, or viable, for our society. It is perfectly reasonable to make sensible, carefully considered policy changes in relation to the types of immigration that we allow and/or the numbers that we might consider to be manageable. It is perfectly reasonable to take humane steps to deal with issues around irregular migration, to tackle the problem of people smuggling or to make changes to how irregular migrants are processed and supported. It is perfectly reasonable to have discussions about the impact of immigration on local communities and to consider steps that can be taken to improve integration etc.
It is not reasonable, imo, to blame migration for all of society's ills; to argue blindly for "net zero" migration without having carefully considered what this might mean for our society; to argue that we should dispense with our commitment to human rights and abandon our obligations under international law. It is not reasonable, imo, to demonise migrants in general, or specific groups of migrants, or to deliberately whip up racial hatred. It is not reasonable, imo, to weaponise violence against women and girls in order to promote inflammatory racist rhetoric. It is not reasonable, imo, to weaponise homelessness amongst British people to promote inflammatory racist rhetoric. It is not reasonable, imo, to try to create fear by promoting ideas such as the Great Replacement Theory. It is not reasonable, imo, to shamelessly exploit the hopelessness of the poor and vulnerable in order to recruit supporters to your cause. It is not reasonable, imo, to spread deliberate disinformation about migrants in order to generate anti-migrant feeling etc.
Hopefully you get the picture.