I guess it could be, but more like if you have bite fetish or lip fetish. I think biting lips during kissing is just inevitable because things get heated up
well, you can call your habit a fetish only if when you do or even imagine it, your sexual desire is getting up. and usually fetish is happening on something that's not typical sexual, so I don't think what happen to you is a fetish, cause biting partner's lips is actually one of way of kissing aggressively (i looked it up on google lol).
but if you tend to bite it hard or maybe your partner feels hurt and bothered about it, there might be another case related to sadistic-masochist thingy. but I doubt it tho.
But like it became really habit of mine even when i only think of us kissing, theres always like biting scenes even in my imagination and also when ready a smut manga, i always look forward to biting lip scene lol.
So umm... is biting your partner's lip when kissing is consider a fetish? Cuz everytime me and my partner kiss, i can't really control/avoid myself from biting my partner's lip.