Filters are a good way to customise your timeline on here. They let you automatically CW or completely hide posts with certain words, phrases or hashtags.
To create a filter on Mastodon:
1. Log in through your server's website (some apps also let you create filters)
2. Click Preferences > Filters (on mobile click
, then ☰, then Filters)
3. Click Add New Filter
4. Choose your settings, then click Save New Filter
There are lots of questions answered about filters at https://fedi.tips/filtering-your-timeline-to-hide-posts-on-mastodon
The filters aren't specifically for hashtags, they're for words which may or may not be in hashtags.
They would have to build a separate hashtag system to have a filter icon next to hashtag follow icons.